Consider making Transporter abstract with all methods returning NotImplementedError

[DanielNoord]

Problem Statement

We used multiple transporter to send to different DSN, this was the previously recommended way to solve this:
#198 (comment)

The new suggestion (#1521) doesn't really work for us as it would be infeasible to start contextmanagers everywhere we use the separate loggers.

However, bumping our sentry-sdk has introduced subtle issues as our TransporterProxy doesn't implement all methods.
Previously all methods were "abstract" but with a6a1be3 they no longer are.

I am asking to make the transporter abstract so that we get better warnings to implement new methods when they get added. I think other users will also benefit from this as it seems like a common use case and users might have followed the same recommendation.

Solution Brainstorm

I am open to making the PR myself but I'd like approval of maintainers first.

[sl0thentr0py]

great suggestion @DanielNoord, thank you!
I agree that making the Transport contract explicit is helpful.

cc @antonpirker whenever we do this, we also need to clean up FunctionTransport since right now it only implements capture_event and is broken.

Also related to Custom Transports.

[sentrivana]

Done in https://github.com/getsentry/sentry-python/pull/2667/files#diff-de9bdd6525bf93f688ebf790909061a7a63fa9a0d3c02443fb6dd2930be8bc42