Add keyspace, shard, and tablet-type/alias filters to ListTablets

[nickvanw]

Summary

Vtctld.ListTablets sent no query parameters, so callers (notably pscale branch list-tablets) had to fetch every tablet on a branch and filter client-side. On large, multi-keyspace branches that makes finding a specific tablet alias painful

Changes

ListBranchTabletsRequest gains four optional filter fields:

• Keyspace, Shard, TabletType — sent as keyspace / shard / tablet_type query parameters.
• TabletAliases []string — sent as a single comma-separated tablet_alias query parameter (matching the api-bb contract). When set, the server ignores the other filters.

Only non-empty filters are added to the query string, so an unfiltered ListTablets call behaves exactly as before. The implementation uses the same WithQueryParams pattern as the other Vtctld methods (ListWorkflows, ListKeyspaces).

Testing

Added vtctld_tablets_test.go (there was no test file before):

• unfiltered request sends no filter params and decodes the grouped response;
• keyspace + shard + tablet_type are sent as the expected query params;
• multiple TabletAliases are serialized into one comma-separated tablet_alias value.

go build, go vet, gofmt, and the full ./planetscale/ test suite pass locally.