Update Conformance logic

[matthewhanson]

Combine Classes STACAPIObjectMixin and Conformance

The STACAPIObjectMixin is a class that takes in a Conformance class to determine conformance. Combine the logic into a single Conformance class that can be added to other classes, currently:

• Client: Used to determine basic required conformance initially
• Search: Used to determine conformance of search endpoints/parameters

Conformance is not currently used, and the code for handling non-conformance needs to be added/updated.

I see this working as follows:

• you can always open any STAC catalog-like object with Client
• Most (all?) functions added in the Client class (which inherits from PySTAC Catalog) should require a conformance check. If non-conformant a ConformanceError is thrown with details.
• If there is no conformsTo array, then assume this is a static catalog. Client will functionally work as a PySTAC Catalog, but most (all?) functions should return a ConformanceErrror
• There should be an override parameter, ignore_conformance to the Client.open() function
• Conformance is also used to determine if certain parameters to the ItemSearch class are allowed. Thus, the ItemSearch class should be passed a ConformanceClass that can be used to check if specific query parameters are allowed. (e.g., if ItemSearch gets a field kwarg and the field conformance is not present, a ConformanceError should be thrown.