[Frontend] Add ability to set new HMAC secret

Why are we doing this work

Current state:

User can set new HMAC secret for external status check:

UI

Required functionality:

1. Once HMAC secret is saved, user can't neither edit nor delete it.
2. Normally, once HMAC secret is saved, it is impossible to edit it
3. But user should at least have a chance to re-write it with new secret

Relevant links

External status checks docs

Non-functional requirements

• Documentation:
• Feature flag:
• Performance:
• Testing:

Implementation plan

1. Go to project
2. Settings -> Merge requests
3. Add new external status check -> save it with secret
4. Open saved external status check -> it should be possible to rewrite secret

Verification steps