Create API module

[jisantuc]

Improvement

#140 adds a bunch of models for interacting with APIs. Some of these are copied directly from Franklin. We should create an API module to hold the shared types and have Franklin and the client depend on those for their client/server implementations instead of maintaining the code in two places.

[pomadchin]

To make it a bit more concrete: we talk here about these models as well, in particular it could be nice to have the following STAC API models:

• PaginationToken (backported into stac4s client)
• Query (backported into stac4s client)
• StacSearchCollection (right now I workaround it via focusing on fields we need for the request)
• CollectionItemsResponse (right now I workaround it via focusing on fields we need for the request)