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/workflows/macos.yml

@@ -128,9 +128,9 @@ jobs:
             configure: --enable-debug --disable-ldap --with-openssl=$(brew --prefix openssl)
             macos-version-min: '10.15'
           # cmake
-          - name: 'OpenSSL gsasl AppleIDN'
-            install: gsasl
-            generate: -DOPENSSL_ROOT_DIR=$(brew --prefix openssl) -DCURL_USE_GSASL=ON -DUSE_APPLE_IDN=ON
+          - name: 'OpenSSL gsasl rtmp AppleIDN'
+            install: gsasl rtmpdump
+            generate: -DOPENSSL_ROOT_DIR=$(brew --prefix openssl) -DCURL_USE_GSASL=ON -DUSE_LIBRTMP=ON -DUSE_APPLE_IDN=ON
           - name: 'OpenSSL +static libssh +examples'
             install: libssh
             generate: -DOPENSSL_ROOT_DIR=$(brew --prefix openssl) -DBUILD_STATIC_LIBS=ON -DCURL_USE_LIBSSH2=OFF -DCURL_USE_LIBSSH=ON