Separate database proto types from external API proto types in Babylon node

[RafilxTenfen]

Currently in Babylon node in API responses we return types directly used in database:

https://github.com/babylonchain/babylon-private/blob/dev/x/btcstaking/keeper/grpc_query.go#L71 which just return delegations in straight from db

https://github.com/babylonchain/babylon-private/blob/dev/x/btclightclient/keeper/grpc_query.go#L71 which return headers as they are stored in db

This is not ideal as:

if we will need to change something in database representation (optimisation/fix/field name change) this will suddenly break our api, which at some point will expected to be stable by external consumers

not everything in db needs to be exposed to users of api.

types for api consumption may be different i.e for example db we can use []byte but in api we perfectly would use hex encoded strings

This story is about improving this situation across every module in Babylon node. The whole migration to make our node API production ready should look roughly:

Create API types in every module and write code translating between transalting db types to api types

After we have proper responses type we need to review them and decide which fields are really necessary and remove the unnecessary ones to have nice clean minimal API. We have large set or programs (dashboard/vigiilantes/reporters) to guide us in this trimming effort.

After we have nicely trimmed api, we may consolidate some practices around it so that endpoints are consistent across different modules.