Part 4: Clarify that default execution mode is at discretion of backend

[m-mohr]

Can the backend decide what the default execution mode is? So there not really any negotiation to get "created" in openEO, which is because there might be operations that you may want to run before actually starting the computation (e.g. estimating the costs).

In openEO it's certainly async, and for OAP it seems sync (which is a bit counterintuitive for me, but that might be because we call jobs actually batch jobs in openEO and batch jobs in synchronous mode feel wrong).

Originally posted by @m-mohr in #437 (comment)

[fmigneault]

As mentioned in #437 (comment), I propose that we define the jobControlOptions order to indicate the default (first element) execution mode. This was never explicitly defined in the standard, but has been mentioned in some past discussions. Servers that want to offer a limited subset of execution modes for a process, can already define the jobControlOptions with supported modes.

Because of the new status: create execution mode of Part 4, we would most probably also need a new create-execute to indicate that as the default (this is what openEO would need to define).

Using the first value by default would allow per-process default execution modes, which I find the most appropriate for granular control over processes with different requirements/implications. My server offers many processes that have different "normal" duration to compute, so a server-wide default is not really meaningful in my case.