Add ability to set partial default in Meta class

Nobody has claimed this yet.

It doesn't seem to be possible to configure a default `partial` property in the `Meta` class. Is there a reason for this?

It would be nice to be able to follow a pattern as follows:

```python
class InsertSchema(Schema):
class Meta:
unknown = EXCLUDE

id = fields.UUID(required=False)
name = fields.Str(required=True)
description = fields.Str(required=True)

class UpdateSchema(InsertSchema):
class Meta(InsertSchema.Meta):
exclude = ["id"]
partial = True
```

The `exclude` in `Meta` works as expected, but the `partial` does not. I'm also using `flask_smorest`, so of course I can define this at the request level, e.g.
```
@blueprint.arguments(UpdateSchema(partial=True))
```
but it would be nice to be able to define it at the class-level, especially if the partial configuration is more complex and perhaps the schema is used in multiple places.

Start by reading the Schema and Meta handling, then compare the class-level partial example for UpdateSchema with the existing request-level partial=True usage. Trace how inherited Meta options such as exclude are processed and add coverage for a schema whose fields become partial by default. Done means the Meta setting works while preserving inherited options.

Written by the indexing model from the issue text.