Tower and Actix: Filter out sensitive headers

[FSMaxB]

I've noticed that when constructing a sentry_core::protocol::Request, all headers are copied even if they are marked as sensitive.

This PR filters out sensitive headers.

[Swatinem]

Thank you, I wasn’t aware so far that some headers are marked as sensitive and there is API for that.

[FSMaxB]

I'm not sure whether any are set as sensitive by default, but there is a middleware in tower-http that can be used to set headers as sensitive for example: https://docs.rs/tower-http/0.5.2/tower_http/sensitive_headers/index.html

[Ten0]

Looks like it might be better to instead write them as <sensitive>, so that people don't start wondering why their headers aren't sent when they in fact are but are just redacted from the logs.