Add the ability to simulate a slow connection

What does this MR do?

With this change we can set the following environment variables, which use Chrome's built-in dev tools network connection settings to simulate a slow connection.

QA_SIMULATE_SLOW_CONNECTION enables the feature when set to true.
QA_SLOW_CONNECTION_LATENCY_MS sets additional latency in ms (default 2000).
QA_SLOW_CONNECTION_THROUGHPUT_KBPS sets the maximum throughput in kbps (default 32).
QA_DISABLE_RSPEC_RETRY will prevent rspec from retrying tests that fail (default false).

See #36015 (closed)

gitlab-qa MR so the variables are passed through: gitlab-qa!330 (merged)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation in gitlab-qa!330 (merged)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content