Add API to manage plugin owned settings

[mpanchajanya]

What this PR does / why we need it

• Provide an API to manage plugin owned settings

Which issue(s) this PR fixes

Fixes #

Describe testing done for PR

• Added unit tests

Manually testing

• Triggered the new API in a test plugin to retrieve the plugins dir

**Test plugin with plugins command**

var pluginsCmd = &cobra.Command{
	Use:   "plugin",
	Short: "Plugin tests",
	Run: func(cmd *cobra.Command, args []string) {
		pluginsDir, err := configlib.GetTanzuPluginConfigDir()
		if err != nil {
			log.Fatal(err, "")
		}
		log.Info(pluginsDir)
	},
}

mpanchajanya@mpanchajanF09MK ~ % tanzu test plugin
[i] /Users/mpanchajanya/.config/tanzu/plugins
mpanchajanya@mpanchajanF09MK ~ %

Release note

Add new API to return config directory to manage plugin owned settings

Additional information

Special notes for your reviewer

[anujc25]

LGTM. Thanks