test: Another step in merging the IGW and router end to end tests

[shmuelk]

What type of PR is this?
/kind test

What this PR does / why we need it:
This PR is an additional step in merging the IGW end to end tests with the llm-d-router end to end tests. With this PR the llm-d-router end to end tests contain all of the end to end tests that are in the IGW's set of end top end tests.

In particular this PR:

• Adds tests of the leadership capabilities of the EPP
• Adds improved metrics tests
• Adds tests of sending encode requests

In addition some refactoring of code used in the end to end tests was done to enable:

• Running multiple replicas of the EPP
• Enabling leader selection
• Making it easier in tests to send batches of requests to test traffic routing

NOTE: This PR does NOT remove the IGW end to end tests. That will happen in a future PR.

Which issue(s) this PR fixes:
Refs: #955

Release note (write NONE if no user-facing change):

NONE