Support Prefer: handling=strict

[fmigneault]

Description

• Support Prefer: handling=strict to enforce a given mode (respond-async or wait)
  For example, using Prefer: handling=strict, wait=5 and failing to complete the process synchronously within the allocated wait time would result in an error rather than switching to asynchronous execution automatically.

• If Prefer: wait=X, handling=strict cannot be respected (since the server is not permitted to fall back to async by strict within the wait time), return "408 Request Timeout".

• For the opposite case, where Prefer: respond-async, handling=strict cannot be respected, respond with "412 Precondition Failed".

• Support Prefer: handling=lenient (default when omitted), that allows automatically switching to async after sync if within wait and respond-async provided simultaneously

• Ensure that allowed execution mode by the process are still respected.
  If they mismatch, directly respond "400 Bad Request".

• Update documentation with Prefer: handling=strict that can be used to enforce (rather than preference only)

  weaver/docs/source/processes.rst

  Lines 670 to 675 in 6f2ac40

  	.. warning::
  	It is important to remember that the ``Prefer`` header is indeed a *preference*. If `Weaver` deems it cannot
  	allocate a worker to execute the task `synchronously` within a reasonable delay, it can enforce the `asynchronous`
  	execution. The `asynchronous` mode is also *prioritized* for running longer :term:`Job` submitted over the task
  	queue, as this allows `Weaver` to offer better availability for all requests submitted by its users.
  	The `synchronous` mode should be reserved only for very quick and relatively low computation intensive operations.

References

• Relates to Default execution mode should be asynchronous. opengeospatial/ogcapi-processes#413