[release-21.0] S3: optional endpoint resolver and correct retrier

[frouioui]

Description

This PR fixes two issues with the AWS S3 client creation that were introduced by github.com//pull/16664.

1. The endpoint resolver option was always passed down to the new client, even if the endpoint flag was not set. That led to the creation of an empty endpoint resolver with an incorrect URL. This issue was discussed in Migrate the S3 SDK from v1 to v2 #16664 (comment) and the fix for it is to conditionally add the endpoint resolver option to the client.
2. After working on Implement a per-file retry mechanism in the BuiltinBackupEngine #17271, I discovered that the AWS retrier was not created correctly leading to ClosedConnectionRetryer.awsRetryer being nil. The fix for this is to correctly create ClosedConnectionRetryer.awsRetryer with the provided functions in the S3 SDK.

This was tested using the same testing infrastructure added to #17271.

Both of these fixes are included in #17271 and backported in this PR as the regression was introduced in v21.0.0.

cc @marf

Related Issue(s)

• Migrate the S3 SDK from v1 to v2 #16664 (comment)

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Please upload report for BASE (release-21.0@985a501). Learn more about missing BASE report.
Report is 3 commits behind head on release-21.0.

Files with missing lines	Patch %	Lines
go/vt/mysqlctl/s3backupstorage/s3.go	0.00%	13 Missing ⚠️

Additional details and impacted files

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  Misses                  ?    81716           
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