E2E Test: escaping secrets in policy

[ycombinator]

Add an E2E test for elastic/kibana#188377, wherein the test should create a policy containing the custom logs integration with a custom configuration like so:

custom.key: $$$$

The test should enroll Agent in Fleet using the created policy and ensure that the configuration sent to the relevant Beat contains custom.key: $$$$ as well.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[kaanyalti]

Blocked due to ESS authentication error required to set up the testing framework