feat: add watchdog stop-switch for bitcoin canister

[maksymar]

This change adds reading and writing bitcoin canister API access config flag.

API access target is calculated based on the height health status, actual value is read from the bitcoin canister. If there is a difference between the target and actual values, then watchdog tries to rewrite the bitcoin canister API access config flag.

This change also exposes additional metrics:

• blocks behind/ahead thresholds
• minimum and actual number of explorers
• API access target

[dsarlis]

Do we have a test to check whether the config can be changed properly? Probably we need an end-to-end test for that?

[maksymar]

Do we have a test to check whether the config can be changed properly? Probably we need an end-to-end test for that?

I'd prefer to separate such an end-to-end test from this PR because it might be a bit too complicated, because to make it truly end-to-end it'd need to intercept HTTP requests from the canister and provide mock responses.

[maksymar]

LGTMs?