Consider supporting a middleware pipeline for IMcpServer

[eiriktsarpalis]

Originally posted by @stephentoub in PederHP/mcpdotnet#82.

Various cross-cutting concerns could be enabled by allowing for a pluggable pipeline, e.g. caching of tool call results, rate limiting, etc.

[stephentoub]

@halter73, do we consider this addressed by the filters?

[halter73]

I do. #733 was the PR that first added filters which was tracked by the "Middleware!" issue (#267).

#761 tracks adding new kinds of middleware/filters that support intercepting all incoming JSON-RPC messages, and maybe all outgoing messages too. It also mentions the possibility of a client middleware pipeline, but we might want to split that out into yet another issuer if there's demand for it.