Allow custom annotations for MCP elements

[Clap404]

I'd like to use custom annotations that extends the ones provided by php-mcp.

I've discovered that some of the annotations I'd like to inherit were final and that the provided Discoverer would not match subclasses of the annotations. I'm not sure if it's accidental or by design.

This change would allow me to use the default Discoverer with custom annotations instead of making my own.

[judus]

Out of curiosity — may I ask what concrete use case you’re trying to support with this?

I’ve been exploring this area myself, and I might be misunderstanding something, but from what I can tell, the MCP server maintains a fairly static registry. Either through attribute discovery or manual registration. The container seems focused on injecting dependencies into tool handlers, not on dynamically filtering or scoping tools — which makes sense, given how shallow PSR-11 is.

So if this PR is about enabling more dynamic resolution, I’m wondering whether this kind of logic really belongs at the attribute level or whether we might eventually need a more expressive registry or tool resolution layer instead?

If the goal is extensibility via attributes, I’d lean toward defining interfaces rather than relying on inheritance. That might offer a cleaner extension point while keeping the existing classes final and predictable.