List all external status checks and their statuses

• What does this MR do?
• Database Review
  • Migrate
  • Rollback
• Walkthrough video
• Does this MR meet the acceptance criteria?
  • Conformity
  • Availability and Testing
  • Security

What does this MR do?

• Adds a sha column to the external_status_checks table of type bytea.
• Adds a REST API endpoint [GET] /projects/:id/merge_requests/:merge_request_iid/status_checks that returns a list of all external approval rules (soon to be renamed external status checks) and whether or not the latest SHA for that merge request has received an approval for it.
• Adds a method approved? to check if a particular external approval rule has received an approval for a particular SHA in an MR.

Database Review

Migrate

== 20210510083845 AddShaToStatusCheckResponse: migrating ======================
-- add_column(:status_check_responses, :sha, :binary, {:null=>false, :default=>""})
   -> 0.0041s
== 20210510083845 AddShaToStatusCheckResponse: migrated (0.0042s) =============

Rollback

== 20210510083845 AddShaToStatusCheckResponse: reverting ======================
-- remove_column(:status_check_responses, :sha)
   -> 0.0029s
== 20210510083845 AddShaToStatusCheckResponse: reverted (0.0030s) =============

Walkthrough video

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
  • Feature is gated, but this MR adds a new migration so requires 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.

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 #328211 (closed)