cmake: add librtmp Find module

The new detection method also allows to enable librtmp without using
OpenSSL as a curl TLS backend at the same time.

Also:
- implement manual version detection for librtmp.
  Version info is in hex. With CMake 3.13 and newer, extract it as a hex
  number. With earlier CMake version, just strip the leading zeroes.
  Doing more here seems overkill because librtmp has been standing
  at 2.3/2.4 for a decade now. Bumping into hex digits seems unlikely
  before deprecating CMake 3.13 support.
  librtmp advertises v2.4 via its `pkg-config` module, and v2.3 via
  its public header. The latter shows up in `curl -V` and either can
  be shown at configure-time depending on detection method.
  This isn't a curl bug.
- GHA/macos: enable rtmp in a job.
- apply the "half-detection" fix to the Find module.
  `librtmp` is also affected (in CI too), because it depends on libssl and
  libcrypto.

Closes #15832

.github/labeler.yml

@@ -377,6 +377,7 @@ RTMP:
   - all:
       - changed-files:
           - any-glob-to-all-files: "{\
+              CMake/FindLibrtmp.cmake,\
               lib/curl_rtmp.*\
               }"