Allow specifying charset in the content-type header

Some HTTP clients always append charset to the content-type header e.g., `content-type: application/json; charset=utf8`. Requests made by those clients are rejected because of the content-type mismatch.

Twirp only allows the content-type header exactly matches one of `aplication/json`, `application/json; strict=true` or `application/probobuf`. This seems too strict, can we relax this constraint?

Thank you.

The issue names no files or tests. Start by locating Twirp's content-type validation and its existing request tests, then compare the accepted media types with clients that append charset. Done means the reported charset-bearing content type is accepted without weakening unrelated content-type checks, with coverage for that header form.

Written by the indexing model from the issue text.