Add the ability to get repository ID and use repositories by ID

[InAnYan]

I haven't dug the GitHub API docs enough, but it seems there is such a thing: https://stackoverflow.com/questions/13902593/how-does-one-find-out-ones-own-repo-id

Getting repo id: https://stackoverflow.com/a/77904644

Accessing by repo ID: https://stackoverflow.com/a/74290453

As a user on StackOverflow said, it's undocumented, but GitHub support said it is supported.

Rationale for this feature:

1. Symmetry: issue_number/issue, id/comment, etc. But repo is accessed by 2 strings
2. No need to pass and handle bluky strings in users code. Just a slim number. (Besides you even require a String, not any other more generic type)
3. It would be easier to write a wrapper around several Git Hosts, as one my use ids extensively, but this GitHub prefers strings of owner/repo..

[InAnYan]

I see you have RepositoryId type and ther is an id type in repository type.

But repository handler is tied up to owner/repo