Move instance provisioning code into Exosphere repository and re-plumb how it's consumed

Fixes #507 (closed). Scope/todo:

• Migrate code from instance-config-mgt repo into Exosphere repo
• Create and document new options in config.js to specify repo and branch/tag/commit of instance provisioning code
• Configure CI jobs to
  • when testing MRs, set instance provisioning repo and commit in config.js (reflecting whatever the MR repo/commit is)
  • when deploying development/production environments, set commit in config.js to ensure instance provisioning code will stay contemporaneous with the Exosphere code that the environment serves
• Document in readme (and any other appropriate places) where instance provisioning code lives and what it's used for
  • Bring relevant parts of readme from instance-config-mgt repo into docs of Exosphere repo
• Turn #507 (closed) into an ADR

How to Test

• Run CI pipeline on GitLab, watch headless browser tests go brrr, they should pass (because test instance finished deploying)
• Deploy a development/production environment (e.g. dev branch), ensure that instanceConfigMgtRepoUrl and instanceConfigMgtRepoCheckout are set appropriately

Screenshots

N/A

Post-Merge Checklist

• Ensure that instanceConfigMgtRepoUrl and instanceConfigMgtRepoCheckout are set appropriately in production environments and that new instances finish deploying
• Deprecate (archive?) exosphere/instance-config-mgt repo
• Create follow-up issue for solution 4 from the description of #507 (closed) (done, #513)