Middleware: Support adding extra instructions/comments to tool description

[ltalirz]

I find it useful to give extra instructions based on the context of a namespace (e.g. which tools to prefer, if no explicit instruction is provided)

MetaMCP already offers the ability to statically override tool descriptions in a particular namespace, which is great.
This has the disadvantage though that updates from the tool server side (e.g. about changes to the underlying tool API) no longer propagate.

It would be great, if one could, for example, simply append extra information to the tool description instead.