Add StringMap flag for altsrc

[dearchap]

My urfave/cli version is

( Put the version of urfave/cli that you are using here )
2.x

Checklist

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

Dependency Management

• My project is using go modules.
• My project is using vendoring.
• My project is automatically downloading the latest version.
• I am unsure of what my dependency management setup is.

Describe the bug

Extend #1580 for altsrc flags

To reproduce

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Observed behavior

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above?

Expected behavior

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Additional context

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Run go version and paste its output here

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[happysnaker]

Follow-up: I implemented the current equivalent on the extracted v3 altsrc module instead of the frozen v2 line. PR: urfave/cli-altsrc#50\n\nIt adds StringMap-compatible value-source support for JSON / YAML / TOML by serializing map values through the same cli.NewStringMap(...).Serialize() path used by urfave/cli/v3. That seemed like the most mergeable place to land this now that new v2 features are off the table.