curl_ed25519: add GnuTLS support (via nettle, hogweed)

The necessary cryptography API is provided by nettle 3.1+, via its
'hogweed' library. The minimum GnuTLS version required by curl is 3.6.5,
which requires nettle 3.4.1+, so the API is always available.

Also:
- autotools: detect and use nettle's hogweed library.
- cmake/FindNettle: add support for the hogweed library.
- GHA/http3-linux: enable in the autotools/cmake GnuTLS jobs.

Ref: 4353ea025a

Closes #22456

.github/workflows/http3-linux.yml

@@ -473,7 +473,7 @@ jobs:
             LDFLAGS: -Wl,-rpath,/home/runner/gnutls/build/lib -Wl,-rpath,/home/runner/nettle/build/lib64 -Wl,-rpath,/home/runner/ngtcp2/build/lib
             PKG_CONFIG_PATH: /home/runner/nettle/build/lib64/pkgconfig:/home/runner/gnutls/build/lib/pkgconfig:/home/runner/nghttp3/build/lib/pkgconfig:/home/runner/nghttp2/build/lib/pkgconfig
             configure: >-
-              --with-gnutls=/home/runner/gnutls/build --with-ngtcp2=/home/runner/ngtcp2/build --with-libssh --enable-ssls-export
+              --with-gnutls=/home/runner/gnutls/build --with-ngtcp2=/home/runner/ngtcp2/build --with-libssh --enable-ssls-export --enable-httpsig
 
           - name: 'gnutls'
             install_packages: libp11-kit-dev libssh-dev
@@ -486,7 +486,7 @@ jobs:
               /home/runner/nghttp2/build/lib/pkgconfig"
             generate: >-
               -DCURL_USE_GNUTLS=ON -DUSE_NGTCP2=ON -DCURL_USE_LIBSSH=ON
-              -DCMAKE_UNITY_BUILD=ON
+              -DCMAKE_UNITY_BUILD=ON -DCURL_DISABLE_HTTPSIG=OFF
 
           - name: 'libressl'
             install_steps: skipall