Add asynchronous and optional opportunity searching

[pjhartzell]

Summary:

This PR follows the proposal for asynchronous opportunity search in this PR branch on the stapi-spec repo by @jkeifer to:

• Make synchronous opportunity search (wait for the http response containing opportunities) optional on each product. A product supports synchronous opportunity search only if it has an opportunity search callable as an attribute.
• Make asynchronous opportunity search (you'll need to check back in a different http request to see if opportunities have been generated yet) optional at the root router level. Support is determined by the presence of certain callables as attributes on the root router and advertising an asynchronous opportunities conformance class. If the root router supports asynchronous opportunity search, all products must support it. If asynchronous opportunity search is supported, POST requests to the /products/{productId}/opportunities endpoint will default to asynchronous opportunity search unless synchronous search is also supported by the product and a Prefer header in the POST request is set to wait.

Note:

• This PR does not add support for updating or accessing asynchronous opportunity search statuses on asynchronous opportunity search records via the API, i.e., it does not add the /searches/opportunities/{searchRecordId}/statuses endpoint for POST and GET requests. Implementations are responsible for modifying statuses. Note that you can always view the current status contained within an asynchronous opportunity search record via the API with a GET to the /searches/opportunities/{searchRecordId} endpoint.

Related Issue(s):

• Add support to ProductRouter for enable/disable of Opportunities endpoints pystapi#31
• Allow opportunities resource search to operate asynchronously stapi-spec#77

Proposed Changes:

1. Makes opportunity searching optional
2. Adds optional asynchronous opportunity searching

PR Checklist:

• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.