Feat: Add value source

[dearchap]

What type of PR is this?

(REQUIRED)

• feature

What this PR does / why we need it:

(REQUIRED)

This places Envs and Filepaths on same footing instead of having one take precedence over the other. In fact the user can choose which to prioritize but placing the sources in the given order. The new interface also allows plugin implementations i.e for reading values from networks, custom format files etc etc.

Which issue(s) this PR fixes:

(REQUIRED)

Special notes for your reviewer:

(fill-in or delete this section)

Testing

(fill-in or delete this section)

Updates all test case

Release Notes

(REQUIRED)

[avorima]

It would be nice to have a convenience constructor for simple cases, e.g.

func EnvVars(...string) []FlagValueSource
func FilePaths(...string) []FlagValueSource

_ = &cli.StringFlag{
        Name: "foo",
        Sources: cli.EnvVars("FOO"),
}

There are a lot of changes in v3 and I think helpers like this would definitely make the transition easier.

[avorima]

👍