rpc: add PeerInfo (#24255)

This replaces the sketchy and undocumented string context keys for HTTP requests
with a defined interface. Using string keys with context is discouraged because
they may clash with keys created by other packages.

We added these keys to make connection metadata available in the signer, so this
change also updates signer/core to use the new PeerInfo API.

rpc/types.go

@@ -40,8 +40,10 @@ type API struct {
 // a RPC session. Implementations must be go-routine safe since the codec can be called in
 // multiple go-routines concurrently.
 type ServerCodec interface {
+	peerInfo() PeerInfo
 	readBatch() (msgs []*jsonrpcMessage, isBatch bool, err error)
 	close()
+
 	jsonWriter
 }