Part 4: Make PATCH /jobs/:id optional

[m-mohr]

In openEO the PATCH operation is optional. Many implementations struggle a bit with implementing updating jobs, especially when already executed. I think I'd recommend to not require this endpoint and make it optional. Could be explored again via a link in GET /jobs/:id (we don't have that link defined in openEO yet, but happy to add).

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