[18.0][MIG] sign_oca_sms: Migration to 18.0

[xaviedoanhduy]

Note:

• Change the values ​​of the state field in the sign.oca.request model in: https://github.com/OCA/sign/pull/119
• Added message = "<p></p>" as a default value to prevent this error: OCA/server-tools – html_text/models/ir_fields_converter.py#L51
• Added test cases.
• Added a readme/ markdown file.
• Added the _rec_names_search attribute and declared the display_name field to enable searching on non-stored fields.
• Add dependency on sms module to avoid KeyError: 'sms.template' error