### What problem does this PR solve?
1. Split SDK class to optimize code structure
`ragflow.get_all_datasets()` ===> `ragflow.dataset.list()`
2. Fixed the parameter validation to allow for empty values.
3. Change the way of checking parameter nullness, Because even if the
parameter is empty, the key still exists, this is a feature from
[APIFlask](https://apiflask.com/schema/).
`if "parser_config" in json_data` ===> `if json_data["parser_config"]`
4. Some common parameter error messages, all from
[Marshmallow](https://marshmallow.readthedocs.io/en/stable/marshmallow.fields.html)
Parameter validation configuration
```
kb_id = fields.String(required=True)
parser_id = fields.String(validate=validators.OneOf([parser_type.value for parser_type in ParserType]),
allow_none=True)
```
When my parameter is
```
kb_id=None,
parser_id='A4'
```
Error messages
```
{
"detail": {
"json": {
"kb_id": [
"Field may not be null."
],
"parser_id": [
"Must be one of: presentation, laws, manual, paper, resume, book, qa, table, naive, picture, one, audio, email, knowledge_graph."
]
}
},
"message": "Validation error"
}
```
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
- fixed documentss API request data schema
- add documents sdk api tests
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
