[API] Create coverage single upload endpoint

[trent-codecov]

Input: Commit SHA (+ slug, token etc)
Output: presigned put url (the standard output from do-upload)

Suggestion:

1. Extract the logic for each of the individual endpoints to a library/service.
2. Call this logic in the individual endpoint.
3. Create new endpoint that sequentially calls each piece.

[trent-codecov]

Step 1 should be gathering requirements from the CLI commands. Ensure that all inputs are handled.

https://github.com/codecov/codecov-cli/blob/main/codecov_cli/commands/upload_process.py