WIP: Introduce optional SiteSpeed pipeline to the GPT

Closes #183 (closed), waiting for performance-sitespeed!1 (merged) to be merged

This MR introduces Rendering Performance Pipeline that could be triggered as an optional job against the reference environments.

To enable this optional job SITESPEED=true environment variable should be set.

• GPT + SiteSpeed pipeline example: https://gitlab.com/gitlab-org/quality/performance-sitespeed/-/pipelines/157748395
• Schedule example: https://gitlab.com/gitlab-org/quality/performance/-/pipeline_schedules/62037/edit
• Wiki report is being sent to quality/performance/-/wikis/Benchmarks/Latest/ENVIRONMENT_NAME/sitespeed, for example: https://gitlab.com/gitlab-org/quality/performance/-/wikis/Benchmarks/Latest/3k/sitespeed

SiteSpeed results example:

TEST                                  | FCP     | SPEED INDEX       | FRONTEND TIME | TRANSFER SIZE | RESULT
--------------------------------------|---------|-------------------|---------------|---------------|-------
web_group                             | 1791 ms | 2049 ms (<2000ms) | 911 ms        | 1215.3 KB     | FAILED
web_project                           | 2789 ms | 3424 ms (<2000ms) | 897 ms        | 1026.2 KB     | FAILED
web_project_branches                  | 1541 ms | 1579 ms (<2000ms) | 693 ms        | 710.6 KB      | PASSED
web_project_commits                   | 1344 ms | 1370 ms (<2000ms) | 984 ms        | 811.3 KB      | PASSED
web_project_file_blame                | 4421 ms | 4440 ms (<2000ms) | 6892 ms       | 1672.8 KB     | FAILED
web_project_file_rendered             | 1011 ms | 8743 ms (<2000ms) | 725 ms        | 1272.8 KB     | FAILED
web_project_file_source               | 1022 ms | 5044 ms (<2000ms) | 740 ms        | 1619.5 KB     | FAILED
web_project_files                     | 1024 ms | 2144 ms (<2000ms) | 830 ms        | 1135.1 KB     | FAILED
web_project_issue                     | 2858 ms | 2922 ms (<2000ms) | 1627 ms       | 1190.9 KB     | FAILED
web_project_issues                    | 2680 ms | 2701 ms (<2000ms) | 857 ms        | 865.3 KB      | FAILED
web_project_merge_request_changes     | 1690 ms | 7657 ms (<2000ms) | 1423 ms       | 2113.2 KB     | FAILED
web_project_merge_request_commits     | 1521 ms | 2801 ms (<2000ms) | 1391 ms       | 1669.3 KB     | FAILED
web_project_merge_request_discussions | 1625 ms | 2337 ms (<2000ms) | 1410 ms       | 1569.9 KB     | FAILED
web_project_merge_requests            | 2155 ms | 2173 ms (<2000ms) | 850 ms        | 832.7 KB      | FAILED
web_project_pipelines                 | 692 ms  | 1869 ms (<2000ms) | 653 ms        | 876.3 KB      | PASSED
web_user                              | 890 ms  | 1777 ms (<2000ms) | 617 ms        | 797.5 KB      | PASSED

When merged

• Update 10k environment schedule with the new SITESPEED variable