sinatra: Add types for req/res and some callbacks

[euglena1215]

No description provided.

[github-actions]

@euglena1215 Thanks for your contribution!

Please follow the instructions below for each change.
See also: https://github.com/ruby/gem_rbs_collection/blob/main/docs/CONTRIBUTING.md

Available commands

You can use the following commands by commenting on this PR.

• /merge: Merge this PR if CI passes

---

sinatra

You changed RBS files for an existing gem.
You can merge this PR yourself because you are a reviewer of this gem.
Just comment /merge to merge this PR.

You can also request a review from other reviewers if you want.

[euglena1215]

/merge