Adds Initial Agentgateway Emitter e2e

[danehans]

What type of PR is this?

/kind bug
/kind cleanup
/kind documentation
/kind test

What this PR does / why we need it:

• Refactors e2e test suite for multiple emitters.
• Cleans-up import groupings.
• Adds agtw emitter readme.
• Updates agtw emitter to no longer provide partial support (agentgateway does not support attaching AgentgatewayPolicy via per-backend HTTPRoute ExtensionRef filters).
• Fiexes bug where agtw emitter was not setting targetRefs (required) in AgentgatewayPolicy.
• Adds initial agtw e2e suite and test cases.

Which issue(s) this PR fixes:

Partially Fixes #59

Does this PR introduce a user-facing change?:

agentgateway emitter: Adds initial e2e suite and test cases; removes partial support (agentgateway does not support attaching `AgentgatewayPolicy` via per-backend `HTTPRoute` `ExtensionRef` filters).