Use `secrecy` crate for credentials

[evdokimovs]

Synopsis

We have credentials fields in Control/Client APIs which should be secret for security purposes. So this PR adds secrecy crate usage for those cases.

Solution

Use secrecy::SecretBox for credentials in the medea-control-api-proto and medea-client-api-proto.

Checklist

• Created PR:
  • [x] In draft mode
  • [x] Name contains issue reference
  • [x] Has type and k:: labels applied
  • [x] Has assignee
• Before review:
  • [x] Documentation is updated (if required)
  • [x] Tests are updated (if required)
  • [x] Changes conform code style
  • [x] CHANGELOG entry is added (if required)
  • [x] FCM (final commit message) is posted or updated
  • [x] Draft mode is removed
• [x] Review is completed and changes are approved
  • [x] FCM (final commit message) is approved
• Before merge:
  • [x] Milestone is set
  • [x] PR's name and description are correct and up-to-date
  • [x] All temporary labels are removed

[evdokimovs]

FCM

Use `secrecy` crate for credentials (#189)