sar_backscatter: get coefficients from schema

[jdries]

OpenEO backends publish their process definitions (schemas), which in many cases follow the official schema, but sometimes not.
The current python client never uses the actual backend schemas, and instead makes assumptions, which leads to user confusion.
It is relatively easy to:

• check if a certain process exists before using it
• check if a certain process argument exists
• check what values are available

One example is the list of supported backscatter coefficients. I suggest to implement metadata lookup, allowing us to improve this check:

openeo-python-client/openeo/rest/datacube.py

Line 2728 in ec66b31

"beta0", "sigma0-ellipsoid", "sigma0-terrain", "gamma0-ellipsoid", "gamma0-terrain", None

Suggestion is to use existing 'describe_process', but add a caching mechanism. Also important to check the case where the process doesn't exist.

openeo-python-client/openeo/rest/connection.py

Line 1051 in ec66b31

def describe_process(self, id: str, namespace: Optional[str] = None) -> dict: