Implement Http2Headers.isEmpty (1.59.x backport)

[ejona86]

Fixes #10665

Using grpc-netty with Netty 4.1.101.Final results in the following error:

io.grpc.StatusRuntimeException: UNKNOWN
	at app//io.grpc.stub.ClientCalls.toStatusRuntimeException(ClientCalls.java:275)
	at app//io.grpc.stub.ClientCalls.getUnchecked(ClientCalls.java:256)
	at app//io.grpc.stub.ClientCalls.blockingUnaryCall(ClientCalls.java:169)
	// Remaining stacktrace omitted
Caused by: java.lang.UnsupportedOperationException
	at io.grpc.netty.AbstractHttp2Headers.isEmpty(AbstractHttp2Headers.java:40)
	at io.netty.handler.codec.http2.DefaultHttp2ConnectionDecoder$FrameReadListener.onHeadersRead(DefaultHttp2ConnectionDecoder.java:419)
	at io.netty.handler.codec.http2.DefaultHttp2ConnectionDecoder$FrameReadListener.onHeadersRead(DefaultHttp2ConnectionDecoder.java:352)
	at io.netty.handler.codec.http2.Http2InboundFrameLogger$1.onHeadersRead(Http2InboundFrameLogger.java:56)
	at io.netty.handler.codec.http2.DefaultHttp2FrameReader$2.processFragment(DefaultHttp2FrameReader.java:476)
	at io.netty.handler.codec.http2.DefaultHttp2FrameReader.readHeadersFrame(DefaultHttp2FrameReader.java:484)
	at io.netty.handler.codec.http2.DefaultHttp2FrameReader.processPayloadState(DefaultHttp2FrameReader.java:253)
	at io.netty.handler.codec.http2.DefaultHttp2FrameReader.readFrame(DefaultHttp2FrameReader.java:159)
	at io.netty.handler.codec.http2.Http2InboundFrameLogger.readFrame(Http2InboundFrameLogger.java:41)
	at io.netty.handler.codec.http2.DefaultHttp2ConnectionDecoder.decodeFrame(DefaultHttp2ConnectionDecoder.java:188)
	at io.netty.handler.codec.http2.Http2ConnectionHandler$FrameDecoder.decode(Http2ConnectionHandler.java:393)
	at io.netty.handler.codec.http2.Http2ConnectionHandler.decode(Http2ConnectionHandler.java:453)
	// Remaining stacktrace omitted

In netty/netty@2657079, Netty introduced code that calls Http2Headers.isEmpty. However the gRPC Http2Headers implementations do not implement this method. I assume this is simply an oversight.

AbstractHttp2Headers feels a fairly brittle. There are a number of methods here that are not implemented by the concrete implementations. Future Netty versions could begin calling these methods and cause similar failures. I wonder if it might be better to eliminate AbstractHttp2Headers to ensure that any unsupported methods are unsupported intentionally rather that accidentally. It seems like this isn't the first time there have been issues like this, see #7953.

Backport of #10663

[SkyeBeFreeman]

When will this bugfix version be released?