Valid media types for links is too restrictive

[mishaschwartz]

The Link class has a type attribute that can either be one of the media types defined by the MimeTypes enum or None

stac-pydantic/stac_pydantic/links.py

Lines 11 to 19 in 68d20aa

	class Link(StacBaseModel):
	"""
	https://github.com/radiantearth/stac-spec/blob/v1.0.0/collection-spec/collection-spec.md#link-object
	"""
	href: str = Field(..., alias="href", min_length=1)
	rel: str = Field(..., alias="rel", min_length=1)
	type: Optional[MimeTypes] = None
	title: Optional[str] = None

However, the MimeTypes enum is missing several common types such as text/xml.

The STAC spec actually does not restrict media types for links to a specific set. It does mention that for asset links that "registered media types are preferred" but the set of registered media types is much much bigger than the MimeTypes enum.

I would suggest changing the Link.type attribute to be simply Optional[str] to conform with the STAC spec.

[fmigneault]

I would like to add that simply adding text/xml would not be a viable solution.
There are many more media-types that would not be supported. Furthermore, more advanced variants of supported ones are problematic as well (e.g.: application/xml;charset=utf-8, application/json;profile=..., etc.).

Optional[MimeTypes|str] would probably be needed to ensure both explicitly referenced enum attributes and other strings are handled. That would align with pystac method.
https://github.com/stac-utils/pystac/blob/7621455aef4813bfe2da571d88484d20bf616122/pystac/link.py#L79

There could also be a more advanced pattern check similar to (?:[a-z]+)\/(?:[a-z]+).* if desired.