`apply()` with kwargs, not only `md`

Currently apply() can only work with a single argument: md.

Suppose we have the following code:

# Extension metadata model
class MyExtensionMetadataModel(BaseModel):
    # Required so that one model can be instantiated with the attribute name
    # rather than the alias
    model_config = ConfigDict(populate_by_name=True)

    # Metadata fields
    name: str = Field(title="Process name", alias=f"{PREFIX}:name")
    authors: List[str] = Field(title="Authors", alias=f"{PREFIX}:authors")
    version: str = Field(title="Process version", alias=f"{PREFIX}:version")


# Create the extension class
MyExtension = create_extension_cls(
    model_cls=MyExtensionMetadataModel,
    schema_uri=SCHEMA_URI
)

Then we only can apply the extension to an item this way:

ext = MyExtension(item)
ext.apply(
  md=MyExtensionMetadataModel(
    name="test",
    authors=["michel", "denis"],
    version="alpha"
 )
)

We should also be able to apply the extension using kwargs, which is more close to pystac extensions:

ext = MyExtension(item)
ext.apply(
  name="test",
  authors=["michel", "denis"],
  version="alpha"
)