Adde enum MediaType for PDF files

[KennSmithDS]

Related Issue(s): #
N/A

Description:
Radiant MLHub team consistently uses PDF files as documentation reference to accompany published datasets. Instead of requiring us to serialize an Item to dictionary to add the custom MediaType and deserialize back to STAC Item Object, adding this Enum type simplifies a process we frequently execute. It's also a common media file type.

PR Checklist:

• Code is formatted (run pre-commit run --all-files)
• Tests pass (run scripts/test)
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.

[guidorice]

@duckontheweb @gadomski @KennSmithDS just an idea: maybe this media type should be broken up into two types?

• documentation PDF
• georeferenced PDF

I did a quick search but cannot tell if georeferenced pdfs have the same or different MIME type.