Add support for (some) SLSTR L2 data types

[simon-sat]

Currently only L1b data from Sen3/SLSTR is supported. This PR adds support for L2 products also.
Initially, only the LST product is supported, but I have made this PR in a flexible way so that others can be added in the future.

In addition, to reduce code duplication I have moved many of the L1b reader functions into a new common library file that can be called by both the L1 and L2 readers.

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

❌ Patch coverage is 98.14815% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.34%. Comparing base (1adb2fa) to head (d1c75b0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
satpy/readers/core/slstr.py	96.96%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3419   +/-   ##
=======================================
  Coverage   96.33%   96.34%           
=======================================
  Files         467      469    +2     
  Lines       59138    59193   +55     
=======================================
+ Hits        56973    57027   +54     
- Misses       2165     2166    +1     

Flag	Coverage Δ
behaviourtests	3.58% <0.00%> (-0.01%)	⬇️
unittests	96.43% <98.14%> (+<0.01%)	⬆️

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