feat: manage session state

[olavloite]

Support SHOW, SET and RESET commands for session variables. The session state
is not used, but all supported SHOW/SET/RESET commands from PostgreSQL are
supported. The transactionality of these statements follow the rules of
PostgreSQL.

[codecov]

Codecov Report

Merging #305 (f042294) into postgresql-dialect (32986c0) will increase coverage by 0.13%.
The diff coverage is 85.89%.

@@                   Coverage Diff                    @@
##             postgresql-dialect     #305      +/-   ##
========================================================
+ Coverage                 83.04%   83.17%   +0.13%     
- Complexity                 1338     1430      +92     
========================================================
  Files                       109      109              
  Lines                      4617     4982     +365     
  Branches                    574      640      +66     
========================================================
+ Hits                       3834     4144     +310     
- Misses                      596      628      +32     
- Partials                    187      210      +23     

Flag	Coverage Δ
all_tests	83.17% <85.89%> (+0.13%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ud/spanner/pgadapter/utils/ClientAutoDetector.java	92.68% <ø> (ø)
...r/pgadapter/statements/SessionStatementParser.java	73.78% <73.78%> (ø)
...gle/cloud/spanner/pgadapter/session/PGSetting.java	91.47% <91.47%> (ø)
...panner/pgadapter/statements/BackendConnection.java	91.19% <95.00%> (-0.07%)	⬇️
.../cloud/spanner/pgadapter/session/SessionState.java	100.00% <100.00%> (ø)
...oud/spanner/pgadapter/statements/SimpleParser.java	92.10% <0.00%> (+0.87%)	⬆️

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