Refactor magic strings to Enum and Constants

Why: avoid bugs and simplify future renamings.

For example we just fixed a bug caused by in one param we sent "e-mail" and in another "email". (Commit: 55bc2c40)

Define module constants or Enums to prevent this kind of bugs, eg:

• Define utility class StrEnum:

# module utils.py

from enum import Enum

class StrEnum(str, Enum):
    """
    Enum where members are also (and must be) strings
    """

    def __str__(self):
        return str(self.value)

• Use StrEnum:

# module data_source.py

class DataSourceResponse(StrEnum):
    USER_ID='user_id'
    EMAIL ='email'
    

from data_source import DataSourceResponse

print(DataSourceResponse.EMAIL)
# prints email

• Or use Constants:

# module data_source.py import DataSource

class DataSource:
    EMAIL ='email'


from data_source import DataSource

print(DataSource.EMAIL)
# prints email

• However usage of enums allows to group constants by meaning, for example:

# module router.py

HEADER_MESSAGE_ID = "message-id"

class MessageInputKeys(StrEnum):
    TARGET='target'
    PRIMARYKEY ='primaryKey'

class MessageKeys(StrEnum):
    CHANNEL_ID='channel_id'
    PRIORITY ='priority'