✨ Add a way to customize applink scheme

[istornz]

This feature intent to add a way to customize app link scheme. By default value are set to "la" as seen in the documentation.

PR related to #41

[ggirotto]

@istornz Just a question: using la as the default value for the scheme wouldn't produce the issue you described here? If we use as an assumption that most of the plugin users don't implement this Plugin <-> Flutter communication (which I personally think are most of the cases), all of these will use the same schema in the end.

TLDR: shouldn't la schema be or custom or null instead providing a default value?

[istornz]

@ggirotto Yes you'r right, I implemented a default value because on the doc, I write "la" & I'm scary about user's who forgot to change on Xcode & on the init ...

But I think I will let the scheme null & precise on the doc how to change it 👍