feat: update ast for supporting generics

[aqaurius6666]

This PR is a follow-up to reverted PR https://github.com/google/wire/pull/360.

• Add a testcase for Generic.

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[aqaurius6666]

@toddtreece Could you help me review this PR 🙇

[osousa]

@stytchiz it seems the test issue referenced here is solved, why not approve this PR and let us have fun with generics and wire? :)