Deprecate `VALIDATOR_REGISTRY_SOURCE = lido`

[vgorkavenko]

The idea is to deprecate lido source and use keysapi as the main validators source for the application.

The action points are:

• add depreciation message if lido is chosen as validator source whenever pulling validators from the source
• add Keys API instance to docker-compose (with corresponding env variables)
• make the corresponding lido env variables optional when keysapi or file is selected