Add filtering support to the v1/connect/intentions endpoint

[mkeeler]

Some of our APIs can accept a filter to limit which results from a listing operation get returned. That is generally documented here.

This feature request is to enable this filtering for the v1/connect/intentions endpoint. The HTTP API should already be parsing the filtering parameters and storing them in the QueryOptions field on the DCSpecificRequest struct. So I think the only work here will be:

• Modify the Intention.List RPC endpoint to enable filtering and add some tests
• Update structs_filtering_test.go with a test for test to assert the filterable fields do not change without us knowing it.
• Update the docs for the HTTP endpoint with the list of filterable fields.