Added design doc for Blaire debug service

[AmHelm]

A first draft for the foreign chain configurations debug service. A lot of feedback is needed. I am aware that the rust functions and structs are incomplete. There are also some comments left in parentheses here and there.

[kevindeforth]

Thanks, this looks really good already!
I left some comments and suggestions.

Slightly off-topic / open-ended discussion points:

• One thing I am curious is what the long-term plan for this service will be. If the goal is just to have access to node-operators configs, then I think we could also turn this on its head:

  • each MPC node serves the config on a restricted endpoint
  • a nearone employee can access the config via that endpoint.

  Access controls will be a bit more complicated though, so that might not be ideal.

• Also, looking at this, I do wonder if we could also use this service for the nodes to fetch information. E.g. we have been discussing with @netrome that we store way too much information on-chain. So, the nodes could use this endpoint to fetch data and then compare its hash to something stored on-chain.

[kevindeforth]

Open question:

• authentication between service and nodes (could be mTLS and we re-use code from backup-service and node, which would require Blaire to have access to the MPC contract state, but it can be fetched via RPC node).
• authentication between service and users (probably, preference SSO via github?)

Remark:
We can pass the IP / web-address of Blaire service to the node in any of the following ways:

• via MPC contract (in which case the address would be public, probably not desired)
• via config-file (probably preferred)