feat: add COPY settings to session state

[olavloite]

Adds COPY settings to the session state so these can be managed using
normal SET/SHOW session commands.

[codecov]

Codecov Report

Merging #338 (1d9325c) into postgresql-dialect (687c952) will increase coverage by 0.05%.
The diff coverage is 98.11%.

@@                   Coverage Diff                    @@
##             postgresql-dialect     #338      +/-   ##
========================================================
+ Coverage                 85.04%   85.10%   +0.05%     
- Complexity                 1648     1669      +21     
========================================================
  Files                       114      115       +1     
  Lines                      5490     5552      +62     
  Branches                    743      754      +11     
========================================================
+ Hits                       4669     4725      +56     
- Misses                      604      606       +2     
- Partials                    217      221       +4     

Flag	Coverage Δ
all_tests	85.10% <98.11%> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
.../cloud/spanner/pgadapter/utils/MutationWriter.java	92.07% <91.66%> (-0.98%)	⬇️
.../cloud/spanner/pgadapter/session/CopySettings.java	100.00% <100.00%> (ø)
.../cloud/spanner/pgadapter/session/SessionState.java	95.70% <100.00%> (-2.89%)	⬇️
...ud/spanner/pgadapter/statements/CopyStatement.java	84.09% <100.00%> (+0.36%)	⬆️

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