Add support context propagation

[He-Pin]

Motivation:
When using the transport, we always need to pass some context from the context to the sessionFactory or handler, but with the current implementation, the original ServerRequest is completely been ignored, it would be nice that we pass this information down.

we have some kind of options:

1. with ThreadLocal
2. With reactor's contextWrite and deferContext
3. with an additional parameter.

I think the 3 is better, which will not bound to any implementation.

[He-Pin]

I'm doing some prototyping, will send a pr later.

[chemicL]

@He-Pin thanks for providing the proposed implementation. I am thinking more about extending the McpServerExchange type to be per-request and encapsulate multiple concerns instead of adding more arguments. The exact shape will reveal itself once we have streamable http for stateless servers, which is coming.

[Randgalt]

This is really needed. Note: because of the use of the Mono/Reactor framework one cannot even use ThreadLocals as internal threads are used for everything. Separately, the use of that framework is a major limitation of this library and should be reconsidered IMO.

[He-Pin]

That's true, without this, we currently have to encode our context in the arguments (path the JSON string, and then extract from the JSON string), which is a huge pain.

[He-Pin]

@chemicL I'm totally fine and thankful if you can get this in the next 0.11.0 release, thanks.

[134130]

@chemicL If you don't mind, I want to take care of it.

[chemicL]

In #420 I added McpTransportContext and an McpTransportContextExtractor. Can you please have a look at it and provide feedback whether something is missing/impossible to achieve with this? There is also #477 now to backport it for SSE.

[He-Pin]

@chemicL Thanks, we are actually starting using it, but I think we should add some type safe method for this, eg:

 public <T> T get(@NonNull final TypedKey<T> key) 

Which is more like Netty's AttributeKey and AttributeMap, which is typesafe.