feat(fastapi): add checked inline type annotations (#4685)

* fastapi: add py.typed marker

PEP 561 requires this marker inside the distributed package to indicate
that downstream users may consume its inline type annotations.

* fix(fastapi): type-check public instrumentation API

Assisted-by: Codex

* fix lint

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Co-authored-by: Leighton Chen <lechen@microsoft.com>
Co-authored-by: aaronabbott <aaronabbott@google.com>