Add _meta property to the main entities in the protocol to support extensibility

[nerzhulart]

MCP has _meta property that can be used for enriching some entities with data that isn't supported by the protocol

[ConradIrwin]

This makes a lot of sense to me, both the idea we can add extra data, and also using the same name as MCP.

If you're building anything with ACP, do you want to send a PR?