add = character to tag validation regex

[cvignal]

The tags restrictions mentioned for the validTagKeyValue regex includes the = character which is not present in the regex.

This PR aims at reducing this drift between AWS and Minio restrictions on this API.
I've added a simple test case in the unit test for this package, let me know if this enough or if further testing is needed.

[harshavardhana]

= is only supported in EC2 not in S3 tags, they don't document it properly.

The regex we have is well tested against AWS S3.

Please provide more evidence, test it with S3 and then let us reopen this PR if needed.

[cvignal]

As per the doc linked in the package:

The allowed characters across all AWS services are: letters (a-z, A-Z), numbers (0-9), and spaces representable in UTF-8, and the following characters: + - = . _ : / @.

In our organization we compute md5 hashes (which can contain =) and store them in tags on S3 so I'm 100% positive that this character is allowed in S3 tags.
I agree that the documentation for S3 is lacking in this regard, I'll open a support case to know the exact rules for validation.

[fleuryc-owkin]

@harshavardhana I confirm : I just successfully manually added a tag containing = to an S3 object via the console.

[harshavardhana]

Please provide a code sample

[klauspost]

When relaxing minio-go restrictions, using mc:

λ go build&&mc tag set --debug mys3/klauspost/test.csv "key=123="
XML:
<Tagging><TagSet><Tag><Key>key</Key><Value>123=</Value></Tag></TagSet></Tagging>
...

mc: <DEBUG> PUT /test.csv?tagging= HTTP/1.1
Host: klauspost.s3.dualstack.us-east-2.amazonaws.com
User-Agent: MinIO (windows; amd64) minio-go/v7.0.77 mc/DEVELOPMENT.GOGET
Content-Length: 80
Accept-Encoding: zstd,gzip
Authorization: AWS4-HMAC-SHA256 Credential=**REDACTED**/20241012/us-east-2/s3/aws4_request, SignedHeaders=content-md5;host;x-amz-content-sha256;x-amz-date, Signature=**REDACTED**
Content-Md5: eqvfVX+z+k3vxTJ6y9/6+w==
X-Amz-Content-Sha256: UNSIGNED-PAYLOAD
X-Amz-Date: 20241012T090131Z

mc: <DEBUG> HTTP/1.1 200 OK
Content-Length: 0
Date: Sat, 12 Oct 2024 09:01:31 GMT
Server: AmazonS3
X-Amz-Id-2: P/u8pn/Q7aFnlKrF7Vy36BrUe4JHZNDiAF2FBLSkg32AiNbj3qEwijahUujyQKdJOc8HN+4+wIWcbp9sSQE+qg==
X-Amz-Request-Id: 1384EV8JC4VX1HVY
X-Amz-Version-Id: null

...

Tags set for https://s3.amazonaws.com/klauspost/test.csv.

λ go build&&mc tag list --debug mys3/klauspost/test.csv
...
mc: <DEBUG> GET /test.csv?tagging= HTTP/1.1
Host: klauspost.s3.dualstack.us-east-2.amazonaws.com
User-Agent: MinIO (windows; amd64) minio-go/v7.0.77 mc/DEVELOPMENT.GOGET
Accept-Encoding: zstd,gzip
Authorization: AWS4-HMAC-SHA256 Credential=**REDACTED**/20241012/us-east-2/s3/aws4_request, SignedHeaders=host;x-amz-content-sha256;x-amz-date, Signature=**REDACTED**
X-Amz-Content-Sha256: UNSIGNED-PAYLOAD
X-Amz-Date: 20241012T090434Z

mc: <DEBUG> HTTP/1.1 200 OK
Transfer-Encoding: chunked
Date: Sat, 12 Oct 2024 09:04:34 GMT
Server: AmazonS3
X-Amz-Id-2: BxvuQLihSZM22S8fbdipp6B+bIxfKex76CFvigSmQyMjYA2FTOibqYtelTEQEmqf8xtalmB3gGSPTJx5mlqbzJlCU2BxvVfRhQ3sJGmQXa0=
X-Amz-Request-Id: R38TVC03WY2HBMS3
X-Amz-Version-Id: null

...

<?xml version="1.0" encoding="UTF-8"?>
<Tagging xmlns="http://s3.amazonaws.com/doc/2006-03-01/"><TagSet><Tag><Key>key</Key><Value>123=</Value></Tag></TagSet></Tagging>

Name : https://s3.amazonaws.com/klauspost/test.csv
key  : 123=

[harshavardhana]

λ go build&&mc tag set --debug mys3/klauspost/test.csv "key=123="

can you try = in the beginning of the string key==123 ?

[klauspost]

λ go build&&mc tag set --debug mys3/klauspost/test.csv "key2==123"
XML:
<Tagging><TagSet><Tag><Key>key2</Key><Value>=123</Value></Tag></TagSet></Tagging>
...
mc: <DEBUG> PUT /test.csv?tagging= HTTP/1.1
Host: klauspost.s3.dualstack.us-east-2.amazonaws.com
User-Agent: MinIO (windows; amd64) minio-go/v7.0.77 mc/DEVELOPMENT.GOGET
Content-Length: 81
Accept-Encoding: zstd,gzip
Authorization: AWS4-HMAC-SHA256 Credential=**REDACTED**/20241012/us-east-2/s3/aws4_request, SignedHeaders=content-md5;host;x-amz-content-sha256;x-amz-date, Signature=**REDACTED**
Content-Md5: fHsUDJyytW3UmybNiuZSsg==
X-Amz-Content-Sha256: UNSIGNED-PAYLOAD
X-Amz-Date: 20241012T093059Z

mc: <DEBUG> HTTP/1.1 200 OK
Content-Length: 0
Date: Sat, 12 Oct 2024 09:30:59 GMT
Server: AmazonS3
X-Amz-Id-2: 1TBaO3Lon0fCK9rewdSdNo5AbuDs5ejvDUUroNJN73GXAyvKrzlw24Av59nbyPISPSFWtKpQZGw+HBTZyQKFOOHJsDGH55ZD8NjG0ZyOJi8=
X-Amz-Request-Id: XYAT0X8YV5TCZBHD
X-Amz-Version-Id: null

λ go build&&mc tag list mys3/klauspost/test.csv
<?xml version="1.0" encoding="UTF-8"?>
<Tagging xmlns="http://s3.amazonaws.com/doc/2006-03-01/"><TagSet><Tag><Key>key2</Key><Value>=123</Value></Tag></TagSet></Tagging>
Name : https://s3.amazonaws.com/klauspost/test.csv
key2 : =123