Allow passing uuid_format to UUID field

• Allow to pass uuid_format (allowed 'hex'(default) or 'string') to UUID field to change the format in which it's saved.
  By default field is saved in hex format (trimmed to 32 chars (without dashes)), but you can pass
  format='string' to use 36 (with dashes) instead to adjust to existing db or other libraries.

  Sample:

  • hex value = c616ab438cce49dbbf4380d109251dce
  • string value = c616ab43-8cce-49db-bf43-80d109251dce