Adding Extension config

[pacostas]

Summary

This PR adds the Extension Config structure similar to what is for Buildpack Config structure https://github.com/paketo-buildpacks/packit/blob/v2/cargo/config.go , but for extensions.

Use Cases

With these changes someone can use the extension config to parse and manipulate extension.toml files. For example the developer can use the NewExtensionParser from the cargo repo to create an ExtensionParser, parse an extension.toml file and manipulate it.

	extensionConfigParser := cargo.NewExtensionParser()
	config, err := extensionConfigParser.Parse(extensionTOMLPath)
	if err != nil {
		return fmt.Errorf("failed to parse extension.toml file : %s", err)
	}

	config.Extension.Version = flags.version

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[pacostas]

I havent added the MarshalJSON and the UnmarshalJSON as I cant see these methods being used somewhere. If that's not the case, let me know to add them with a further commit :)

[robdimsdale]

This looks good to me. I'd like one more tooling maintainer to take a look and if they're happy with it this can be merged.

For posterity: I was initially unsure of the value of creating a separate config, but I do see that some fields are different (e.g. there's the top-level [extension] object) so I think this change makes sense. I don't think it's valuable to try to use generics for this - they're separate objects with separate fields. If we find there is complex behavioral overlap in the future we can explore generics.