net/http: add basic unexported pluggable HTTP/3 support

Following #77440, this CL adds a basic support for plugging in an HTTP/3
implementation to net/http. As the proposal is not accepted yet, this CL
does not add any exported symbols.

Access to plug HTTP/3 support is locked behind
net/http.protocolSetHTTP3, which can only be used via linkname by
golang.org/x/net/internal/http3_test.protocolSetHTTP3. This will allow
us to run our HTTP/3 implementation in x/net againts various tests in
net/http to support development, without expanding the API surface for
any users.

Support for closeIdleConnectionser will be added separately in the
future.

For #77440

Change-Id: I6e3a0c2e9b329cef43e4682463ed5e2093d04256
Reviewed-on: https://go-review.googlesource.com/c/go/+/740120
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

0 files changed, 0 insertions, 0 deletions