HTTP3: initial (experimental) support

USe configure --with-ngtcp2 or --with-quiche

Using either option will enable a HTTP3 build.
Co-authored-by: Alessandro Ghedini <alessandro@ghedini.me>

Closes #3500

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+# HTTP3 (and QUIC)
+
+## Resources
+
+[HTTP/3 Explained](https://daniel.haxx.se/http3-explained/) - the online free
+book describing the protocols involved.
+
+[QUIC implementation](https://github.com/curl/curl/wiki/QUIC-implementation) -
+the wiki page describing the plan for how to support QUIC and HTTP/3 in curl
+and libcurl.
+
+[quicwg.org](https://quicwg.org/) - home of the official protocol drafts
+
+## QUIC libraries
+
+QUIC libraries we're experiementing with:
+
+[ngtcp2](https://github.com/ngtcp2/ngtcp2)
+
+[quiche](https://github.com/cloudflare/quiche)
+
+## Experimental!
+
+HTTP/3 and QUIC support in curl is not yet working and this is early days.
+Consider all QUIC and HTTP/3 code to be **EXPERIMENTAL** until further notice.
+
+curl does not have HTTP/3 support (yet).
+
+The bleeding edge QUIC work is done in the dedicated
+[QUIC](https://github.com/curl/curl/tree/QUIC) branch, but the plan is to
+merge as often as possible from there to master. All QUIC related code will
+remain being build-time conditionally enabled.
+
+# ngtcp2 version
+
+## Build
+
+1. clone ngtcp2 from git (the draft-17 branch)
+2. build and install ngtcp2's custom OpenSSL version (the quic-draft-17 branch)
+3. build and install ngtcp2 according to its instructions
+4. configure curl with ngtcp2 support: `./configure --with-ngtcp2=<install prefix>`
+5. build curl "normally"
+
+## Running
+
+Make sure the custom OpenSSL library is the one used at run-time, as otherwise
+you'll just get ld.so linker errors.
+
+## Invoke from command line
+
+    curl --http3-direct https://nghttp2.org:8443/
+
+# quiche version
+
+## build
+
+Build BoringSSL (it needs to be built manually so it can be reused with curl):
+
+     % mkdir -p quiche/deps/boringssl/build
+     % cd quiche/deps/boringssl/build
+     % cmake -DCMAKE_POSITION_INDEPENDENT_CODE=on ..
+     % make -j`nproc`
+     % cd ..
+     % mkdir .openssl/lib -p
+     % cp build/crypto/libcrypto.a build/ssl/libssl.a .openssl/lib
+     % ln -s $PWD/include .openssl
+
+Build quiche:
+
+     % cd ../..
+     % QUICHE_BSSL_PATH=$PWD/deps/boringssl cargo build --release
+
+Clone and build curl:
+
+     % cd ..
+     % git clone https://github.com/curl/curl
+     % ./buildconf
+     % ./configure --with-ssl=$PWD/../quiche/deps/boringssl/.openssl --with-quiche=$PWD/../quiche --enable-debug
+     % make -j`nproc`
+
+## Running
+
+Make an HTTP/1.1 request to a QUIC server:
+
+     % src/curl --http3-direct https://cloudflare-quic.com/