Schemas update

Schemas package update:

• refactor get_schema method:
  • improve error description -- add path to file which should be edited,
  • add type annotations,
• no need complement __init__ file. Method get_schema imports entire contents of the file response_schema.py.
• repair PublicKey:
  • accept only Base58 characters,
  • change valid lengths.
• add the ability to use parts of schemas: AssetHbd, AssetHive, AssetVests. E.g.:
  • AssetHBD.Amount(),
  • AssetHBD.Precision(),
  • AssetHBD.Nai().
• created custom schemas: Legacy assets, Hex, Signature, AccountName, Permlink, TransactionId, Json.

Part of changes in this merge request was reviewed in previous merge request: !3 (merged)

To deliver these changes to hive in one delivery, all prepared changes were merged to 2mk-common-branch.