[pluggable bbr] add tests for testing request headers setting/remove by multiple different plugins

[nirrozenbaum]

with the merge or PR #2513, we need to add unit-tests and integration tests to make sure setting/removing headers via request plugins are working as expected in all combinations.

some example for use cases to test:

• a first plugin adds a header, second plugin removes the same header - end result: nothing changed.
• a first plugin adds a header, second plugin removes a different header - both changes apply.
• a plugin removes an non-existing header - nothing changes.
• a plugin removes a header and second plugin is setting it with a new value - the new value is updated.
• etc.

[noalimoy]

/assign