Add support for alias in YAMLs

[james-prysm]

What type of PR is this?

(REQUIRED)

• bug

What this PR does / why we need it:

Currently, one can not process aliased flags via yaml config file because only the original name is used in the flag lookup.

Which issue(s) this PR fixes:

Fixes #1394

Special notes for your reviewer:

First time trying to create a PR.
Supports all flag types defined in flag.go

Testing

Unit Tested

Release Notes

Supports the setting of flags through an alias name via yaml config file

[Dokiys]

@dearchap @james-prysm Do you think there is too mush nesting? Should we return early when don't need to handle. For example:

	if f.set == nil || cCtx.IsSet(f.Name) || isEnvVarSet(f.EnvVars) {
		return nil
	}
	for _, name := range f.BoolFlag.Names() {
		if !isc.isSet(name) {
			continue
		}
		value, err := isc.Bool(name)
		if err != nil {
			return err
		}
		for _, n := range f.Names() {
			_ = f.set.Set(n, strconv.FormatBool(value))
		}
	}

[dearchap]

@Dokiys definitely your approach is more readable. You want to make the changes and push ?