Add a start-up validation for relay's operator existence

[natanasow]

Problem

Currently, we can start a relay with a non-existing account for the operator.

Solution

Add a start-up validation to validate whether the operator exists.

The check can be done via:

• payable (~0.002 HBARs) SDK query to getAccountInfo - this way we will check that pair operatorId - operatorKey is valid and the operator can submit transactions
• mirror node call to get an account by id - this way we will make sure that operator id exists but we can't know if the operator key is valid

Alternatives

No response