northbound: add max_depth and exclude filters for state retrieval

[sha90w]

Extend the GetRequest API with two filters that apply to state data:

- max_depth: limits subtree traversal to N levels beyond the requested target. Encoded on the gRPC wire as a uint32 on Path (0 = unlimited);

- exclude: a list of paths whose subtrees are skipped. A single-element pattern matches by name at any depth; multi-element patterns match a contiguous tail of the data path. Choice/case ancestors are skipped during matching so excludes mirror the data tree shape.