feat: add support for backend-specific parameters

[emmanuelmathot]

Summary

Adds **kwargs support to load_collection() to allow passing additional backend-specific parameters that are not part of the standard openEO specification.

Motivation

Different openEO backends may support additional parameters like nodata, target_crs, tile_buffer, etc. This change enables users to leverage these backend-specific features.

Example

cube = connection.load_collection(
    "sentinel-2-l2a",
    spatial_extent={"west": 10, "south": 45, "east": 11, "north": 46},
    temporal_extent=["2023-01-01", "2023-06-01"],
    bands=["B04", "B03", "B02"],
    nodata=0,           # backend-specific
    target_crs=32632,   # backend-specific
)

Changes

• Added **kwargs to Connection.load_collection() and DataCube.load_collection()
• Extra parameters are passed through to the process graph
• Added tests and updated documentation

[soxofaan]

quick note: can you avoid unrelated code style tweaks in this PR, as it makes it near impossible to see what actually changed here

[soxofaan]

Also note, there is already a helper to append non-standard arguments
documented at https://open-eo.github.io/openeo-python-client/processes.html#advanced-argument-tweaking

# `Connection.load_collection` does not support `feature_flags` argument
cube = connection.load_collection(...)

# Add `feature_flag` argument `load_collection` process graph node
cube.result_node().update_arguments(feature_flags="rXPk")

note that this is a more general solution, not limited to load_collection nodes
and it more clearly flags which parts of your code are outside of the standard spec