Persist projects configured to use an Agent

What does this MR do?

• Creates a new model to represent projects that have been configured in an Agent configuration project.
• Expands the existing auth refreshing logic (currently works in the same way, but at the group level) to include authorized projects.

Further details can be found in !68023 (merged), which added the same model/logic for the group level functionality.

Migrations

== 20210913010411 CreateAgentProjectAuthorizations: migrating =================
-- create_table(:agent_project_authorizations)
   -> 0.0268s
== 20210913010411 CreateAgentProjectAuthorizations: migrated (0.0268s) ========

== 20210913010432 AddAgentProjectAuthorizationsForeignKeys: migrating =========
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:agent_project_authorizations)
   -> 0.0044s
-- execute("ALTER TABLE agent_project_authorizations\nADD CONSTRAINT fk_1d30bb4987\nFOREIGN KEY (project_id)\nREFERENCES projects (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0046s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- execute("ALTER TABLE agent_project_authorizations VALIDATE CONSTRAINT fk_1d30bb4987;")
   -> 0.0099s
-- execute("RESET statement_timeout")
   -> 0.0014s
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:agent_project_authorizations)
   -> 0.0047s
-- execute("ALTER TABLE agent_project_authorizations\nADD CONSTRAINT fk_b7fe9b4777\nFOREIGN KEY (agent_id)\nREFERENCES cluster_agents (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0024s
-- execute("ALTER TABLE agent_project_authorizations VALIDATE CONSTRAINT fk_b7fe9b4777;")
   -> 0.0075s
== 20210913010432 AddAgentProjectAuthorizationsForeignKeys: migrated (0.0481s)

== 20210913010432 AddAgentProjectAuthorizationsForeignKeys: reverting =========
-- foreign_keys(:agent_project_authorizations)
   -> 0.0064s
-- remove_foreign_key(:agent_project_authorizations, {:column=>:project_id})
   -> 0.0068s
-- foreign_keys(:agent_project_authorizations)
   -> 0.0029s
-- remove_foreign_key(:agent_project_authorizations, {:column=>:agent_id})
   -> 0.0049s
== 20210913010432 AddAgentProjectAuthorizationsForeignKeys: reverted (0.0556s)

== 20210913010411 CreateAgentProjectAuthorizations: reverting =================
-- drop_table(:agent_project_authorizations)
   -> 0.0056s
== 20210913010411 CreateAgentProjectAuthorizations: reverted (0.0116s) ========

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #323708 (closed)

Closes #327411 (closed)