Make openeo-processes version more explicit

In openEO there is no direct coupling between openeo API version (now at 1.2) and openeo processes version (now at 2.0).

A user has at most control over the API version they want, e.g. through the API endpoint they connect to (e.g. https://openeo.vito.be/openeo/1.1 or https://openeo.vito.be/openeo/1.2). However, last week we experienced that the processes version is quite relevant too (e.g. user was using text_merge which was renamed to text_concat` in 2.0).

Related: in #195 we also had the problem of how to introduce the 2.0 version of processes in the VITO backend, without breaking existing usage and decided to link processes 2.0 to api version 1.2.

Some ideas to improve the situation:

• more explicitly expose what processes version is is available, e.g. through the capabilities doc
• allow user to explicitly pick the desired process version, e.g. through the connection url: /openeo/1.2-2.0 would be api 1.2 with processes 2.0 ?

Start by reviewing related issue #195 and the capabilities document mentioned in this issue. Clarify whether the desired change is to expose the available processes version, allow explicit selection through the connection URL, or both. Done means the chosen versioning behavior and compatibility expectations are specified.