fix: remove default for type field of the Attachment class

[adubovik]

The SDK defaults a missing type field to text/markdown:

ai-dial-sdk/aidial_sdk/chat_completion/request.py

Lines 21 to 27 in 2835107

	class Attachment(ExtraForbidModel):
	type: Optional[StrictStr] = "text/markdown"
	title: Optional[StrictStr] = None
	data: Optional[StrictStr] = None
	url: Optional[StrictStr] = None
	reference_type: Optional[StrictStr] = None
	reference_url: Optional[StrictStr] = None

This non-trivial default makes it impossible to receive attachments like this one:

{ "url": "https://example.com/foo/bar/doc.pdf"}

Because it will be parsed to the object:

Attachment(url="https://example.com/foo/bar/doc.pdf", type="text/markdown")

which is utterly bizarre and hard to deal with in the applications.
See how it's worked around in the adapter-bedrock, likewise in adapter-vertexai.

Essentially, the parser from JSON to Attachment

1. reinterprets the data from the user as something that it isn't, and
2. obscures the data, because the downstream code doesn't see what exactly came in the request.

The proposal is to default to None.

On the other hand, SDK provides inconsistent ways to create Attachment objects.

See helpers at aidial_sdk.utils._attachment:

>>> print(create_attachment(Attachment()))
type='text/markdown' title=None data=None url=None reference_type=None reference_url=None
>>> print(create_attachment())
type=None title=None data=None url=None reference_type=None reference_url=None