Coordinator API - endpoint for update log signature

[fnerdman]

Use case

Attestation in air gapped environments once the coordinator has received an update.

Describe your solution

Similar to the #291 PR, the update endpoint should return a signature of the update logs - signed by the intermediate key

Additional context

#290

[m1ghtym0]

@lead4good Thanks! Are you working on a PR for this feature?

[fnerdman]

@brenzi @m1ghtym0 No PR is in work at this moment. For Integritee the priority is #293. Only after this a valid signature for the update log will be useful.