cmake: add and use local FindGnuTLS module

Replacing a combination of custom logic in the main script and relying
on CMake's built-in Find module, with code and behavior used for
the rest of dependencies.

Also to:
- add version detection in the non-pkg-config path.
- make `GNUTLS_INCLUDE_DIR` and `GNUTLS_LIBRARY` take precedence over
  pkg-config. As with other dependencies.
- document the above two configuration options.
- prepare for #16973, which originally introduced this local Find
  module.

The local module is doing largely the same as CMake's built-in
FindGnuTLS. Differences:
- honors `CURL_USE_PKGCONFIG`.
- returns GnuTLS version for non-pkg-config detection.
- consistently returns `GNUTLS_VERSION`.
  (CMake's built-in uses s different name in <3.16.)
- CMake 3.16+ returns an imported target. curl supports 3.7,
  therefore we may only use it conditionally, which isn't worth it.

Cherry-picked from #16973

Closes #19163

docs/INSTALL-CMAKE.md

@@ -404,6 +404,8 @@ Details via CMake
 - `CARES_INCLUDE_DIR`:                      The c-ares include directory.
 - `CARES_LIBRARY`:                          Path to `cares` library.
 - `DL_LIBRARY`:                             Path to `dl` library. (for Rustls)
+- `GNUTLS_INCLUDE_DIR`:                     The GnuTLS include directory.
+- `GNUTLS_LIBRARY`:                         Path to `gnutls` library.
 - `GSS_ROOT_DIR`:                           Set this variable to the root installation of GSS. (also supported as environment)
 - `LDAP_INCLUDE_DIR`:                       The LDAP include directory.
 - `LDAP_LIBRARY`:                           Path to `ldap` library.