test: introduce api search e2e test fixture

[ashtonG]

Ticket

ET-821

Description

this introduces an e2e api fixture that returns objects that allow for api control of experiments. it should be useful for quickly scaffolding state before tests as well as imperatively updating the server state during tests. This should fulfill ET-821 in that the fixture can be used to create experiments with trials as well as report metrics, checkpoints, and logs for the trial.

We also modify the existing api fixtures to:

1. only get the token cookie once per worker/test (depending on the lifetime of the fixtures)
2. reuse the api class between calls in the fixture
3. centralize api instantiation as well as url normalization

Test Plan

No tests required

Checklist

• [x] Changes have been manually QA'd
• [x] New features have been approved by the corresponding PM
• [x] User-facing API changes have the "User-facing API Change" label
• [x] Release notes have been added as a separate file under docs/release-notes/ See Release Note for details.
• [x] Licenses have been included for new code which was copied and/or modified from any external code