cloud service query-endpoint create should be additive

[sdairs]

Problem

The instance_query_endpoint_upsert API endpoint is an upsert — calling cloud service query-endpoint create --open-api-key K2 on a service that already has K1 authorized replaces the config, so K1 loses access.

Discovered while working on #115 (the new cloud service query command). A user adding a second key expects to keep the first.

Proposed behaviour

cloud service query-endpoint create should:

1. First fetch the existing endpoint (if any).
2. Union the provided --open-api-key, --role and --allowed-origins values with what's already there.
3. Upsert the merged config.

Add a --replace flag for the current clobber behaviour. Alternatively rename `create` to `upsert` or add a sibling `update` that is explicitly additive.

Related

• rewrite cloud service query to use query endpoints #115 — introduced the new query command and exposed this footgun.