Improve chunking for Xarray generated with the earthkit engine

[sandorkertesz]

This PR adds the following changes:

• improves the serialisation of GRIB fieldlists used when Xarray is generated with chunks
• a new config option grib-file-serialisation-policy was added to control how GRIB data on disk is pickled. The options are "path" and "memory". The default is "path". Previously, only "memory" was implemented.
• serialisation was added to GRIB fields (both on disk and in-memory)
• serialisation was added to the FieldListTensor
• TensorBackendArray, which implements the lazy loading of DataArrays in the Xarray engine, now uses a dask.utils.SerializableLock for data loading

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.53086% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.88%. Comparing base (1361f58) to head (610ddaa).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
tests/xr_engine/test_xr_builder.py	97.01%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #700      +/-   ##
===========================================
+ Coverage    90.80%   90.88%   +0.08%     
===========================================
  Files          164      165       +1     
  Lines        12453    12533      +80     
  Branches       609      612       +3     
===========================================
+ Hits         11308    11391      +83     
+ Misses         964      960       -4     
- Partials       181      182       +1     

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