…eed to define (de)serialization in the same place

As a novice attrs user I was confused about the factory= parameter,
which is just syntatic sugar for a default with an attr.Factory. The
default=attr.Factory is used in other similar scenarios, and this
aligns with the other examples for readability

Include eo etension in collection as the summary entries reference it

Otherwise this throws a JSON serialization error

This is done by the tile extension to perform a redirect

This logic was previosly just in the transaction extension, but is
required across all deserializations

TypedDicts require that either all attributes are considered required,
or None. Because STAC has optional attributes that we don't want users
to put "None" into (and therefore make the serialized responses have
"null" values in fields, which produces invalid STAC), it's better to
allow all fields to be optional. This allows users to utilize the type
information in the TypedDict when constructing STAC objects, but
not be told by type hinting to supply optional fields they don't
have. This produces some type errors around direct dict access to
fields of the potential for the dict key to not be present.

This commit adds pystac as a development dependency, and adds tests
around collection and item fetching to validate the output of the API routes

Required changing the Item to make it valid for EO, which included
removing existing invalid bands objects from properties and setting on
the assets. Did this via grabbing example JSON from a Planetary Computer
landsat-8-c2-l2 Item