Add StringMap flag

[avorima]

Checklist

• Are you running the latest v2 release? The list of releases is here.
• Did you check the manual for your release? The v2 manual is here.
• Did you perform a search about this feature? Here's the GitHub guide about searching.

What problem does this solve?

There is no string mapping flag so I usually end up using a string slice and then splitting the elements into key/value pairs. pflag has the option to use the StringToString flag: https://github.com/spf13/pflag/blob/master/string_to_string.go.

Solution description

Add a StringMapFlag and StringMap type that can be used like StringSlice/Flag, but is backed by a map[string]string and splits the elements accordingly with validation. Separator could be configurable, but I think = is a common default.

Describe alternatives you've considered

Create a custom flag, but that would be even more boilerplate than now.

[dearchap]

@avorima We are not adding any new features to v2. Would you like to submit a PR for this for v3 ?

[avorima]

Yeah, I can give it a shot. So do I use main or v3-dev-main as base? Looking at https://cli.urfave.org/CONTRIBUTING/ I guess it should be v3-dev-main, but I see that main already has v3 in its go.mod

[dearchap]

yes please use main.

[artemklevtsov]

Will there be support for altsrc?

[dearchap]

@artemklevtsov would you like to submit a PR for altsrc ?

[artemklevtsov]

Hm... I'll digging into it.

[dearchap]

@artemklevtsov Lets continue this conversation in issue #1634