cmake: add CMake Config-based dependency detection

After limiting `find_package()`/`find_dependency()` calls to curl local
Find modules via the `MODULES` keyword, it became possible to detect
dependencies via CMake Configs from within those local Find modules, by
calling `find_package()` again with the `CONFIG` keyword. This patch
implements this. Then maps detection results to the result variables and
curl-specific imported targets the rest of the build expects.

Also honor recently introduced `*_USE_STATIC_LIBS` (experimental) flags
to map to the static target when requested.

This adds CMake Configs as an alternative to the existing `pkg-config`
and `find_path()`/`find_library()` auto-detection methods.

Enabled by default for MSVC, outside vcpkg and when not cross-building.
To enable for other cases, or override the default, you can use
`-DCURL_USE_CMAKECONFIG=ON` or `OFF`.

When enabled, Config detection happens after `pkg-config` and before
`find_path()`/`find_library()`. Using CMake's built-in options, you may
also manually point to the absolute directory holding Config files:

`Libssh2_DIR`, `MbedTLS_DIR`, `NGHTTP2_DIR`, `NGHTTP3_DIR`,
`NGTCP2_DIR` v1.19.0+ (with non-fork OpenSSL only), `Zstd_DIR` v1.4.5+

E.g. `-DMbedTLS_DIR=/path/to/mbedtls/lib/cmake/MbedTLS`

These dependencies typically need to be built with CMake to support
this.

Tagged as experimental.

Refs:
#20013 #19156 #19117
https://github.com/curl/curl/pull/20784#issuecomment-3984318492

Depends-on: fad1ebaecc #20840
Follow-up to 91e06fde1b #20784
Follow-up to 26c39d8df1 #20015

Closes #20814

tests/cmake/CMakeLists.txt

@@ -92,7 +92,8 @@ elseif(TEST_INTEGRATION_MODE STREQUAL "ExternalProject")
     URL "${FROM_ARCHIVE}" URL_HASH "SHA256=${FROM_HASH}"
     ${_download_extract_timestamp}
     PREFIX "${CMAKE_BINARY_DIR}/curl-external"
-    CMAKE_ARGS "-DCMAKE_INSTALL_PREFIX=${_curl_install_dir}" -DBUILD_SHARED_LIBS=OFF -DCURL_USE_LIBPSL=OFF -DCURL_USE_PKGCONFIG=OFF
+    CMAKE_ARGS "-DCMAKE_INSTALL_PREFIX=${_curl_install_dir}" -DBUILD_SHARED_LIBS=OFF -DCURL_USE_LIBPSL=OFF
+      -DCURL_USE_CMAKECONFIG=OFF -DCURL_USE_PKGCONFIG=OFF
       -DCURL_ENABLE_SSL=OFF -DCURL_ENABLE_SSL=OFF -DCURL_DISABLE_LDAP=ON -DCURL_USE_LIBSSH2=OFF -DUSE_NGHTTP2=OFF
       -DCURL_BROTLI=OFF -DCURL_ZLIB=OFF -DCURL_ZSTD=OFF -DUSE_LIBIDN2=OFF -DENABLE_IPV6=OFF
       ${CURL_TEST_OPTS}