Introduce Fix.session to access Session in fixes

[schlunma]

Description

To make fixes even more flexible I added a session attribute which includes the current Session. Example use case is loading additional data in the fix from the auxiliary_data_dir, which can now be accessed via Fix.session['auxiliary_data_dir'].

Closes #1986

Link to documentation: https://esmvaltool--1988.org.readthedocs.build/projects/ESMValCore/en/1988/develop/fixing_data.html#fix-structure

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[codecov]

Codecov Report

Merging #1988 (e1656dd) into main (88827d6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1988   +/-   ##
=======================================
  Coverage   92.79%   92.79%           
=======================================
  Files         236      236           
  Lines       12484    12485    +1     
=======================================
+ Hits        11585    11586    +1     
  Misses        899      899           

Impacted Files	Coverage Δ
esmvalcore/dataset.py	100.00% <ø> (ø)
esmvalcore/cmor/_fixes/fix.py	98.57% <100.00%> (-0.03%)	⬇️
esmvalcore/cmor/fix.py	100.00% <100.00%> (ø)

[valeriupredoi]

two small comments that don't warrant a rejection 😁 Cheers, bud 🍺