[API] Connector exists method should be lightweight

[jedrazb]

Problem Description

We currently check if connector exists (e.g. for agentless connectors component) by calling GET _connector/{id}, this in turn fetches all doc source. If we have a big cursor then it might turn into an expensive call.

See this comment: #2923 (comment)

Proposed Solution

Connector API should support making it more efficient to check if a connector exists

Alternatives

Keep it as it is... always fetch the whole object