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

docs/INSTALL-CMAKE.md

@@ -347,6 +347,7 @@ Details via CMake
 ## Dependencies
 
 - `CURL_BROTLI`:                            Use brotli (`ON`, `OFF` or `AUTO`). Default: `AUTO`
+- `CURL_USE_CMAKECONFIG`:                   Enable detecting dependencies via CMake Config. Default: `ON` for MSVC (except under vcpkg), if not cross-compiling. (experimental)
 - `CURL_USE_GNUTLS`:                        Enable GnuTLS for SSL/TLS. Default: `OFF`
 - `CURL_USE_GSASL`:                         Use libgsasl. Default: `OFF`
 - `CURL_USE_GSSAPI`:                        Use GSSAPI implementation. Default: `OFF`
@@ -383,6 +384,11 @@ Details via CMake
 - `ZLIB_INCLUDE_DIR`:                       Absolute path to zlib include directory.
 - `ZLIB_LIBRARY`:                           Absolute path to `zlib` library.
 - `ZLIB_USE_STATIC_LIBS`:                   Look for static `zlib` library (requires CMake v3.24).
+- `<PackageName>_DIR`:                      Absolute path to `<PackageName>` CMake Config directory where `*.cmake` files reside. Used when `CURL_USE_CMAKECONFIG` is enabled.
+                                            `<PackageName>` may be:
+                                            `Libssh2`, `MbedTLS`, `NGHTTP2`, `NGHTTP3`,
+                                            `NGTCP2` 1.19.0+ (with non-fork OpenSSL only),
+                                            `Zstd` 1.4.5+.
 
 ## Dependency options (tools)