feat: support \l meta command

[olavloite]

Adds support for the \l psql meta command to list all databases on the same 'server'. The meta command behaves as follows in PGAdapter:

1. If PGAdapter has been started with -d (a fixed database) it will only show the current database, as no other database can be 'reached' by using \c.
2. If -d has not been specified, PGAdapter shows all the databases on the same instance as the current connection.
3. The databases in step 2 are shown with only the database id if the current connection is to the default instance of this instance of PGAdapter. Otherwise, the fully qualified name is shown.

[codecov]

Codecov Report

Merging #215 (cbd6d9b) into postgresql-dialect (087cb1d) will increase coverage by 0.15%.
The diff coverage is 87.87%.

@@                   Coverage Diff                    @@
##             postgresql-dialect     #215      +/-   ##
========================================================
+ Coverage                 80.56%   80.72%   +0.15%     
- Complexity                 1051     1067      +16     
========================================================
  Files                        91       92       +1     
  Lines                      3556     3647      +91     
  Branches                    426      431       +5     
========================================================
+ Hits                       2865     2944      +79     
- Misses                      525      537      +12     
  Partials                    166      166              

Flag	Coverage Δ
all_tests	80.72% <87.87%> (+0.15%)	⬆️

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

Impacted Files	Coverage Δ
...ud/spanner/pgadapter/metadata/OptionsMetadata.java	78.78% <0.00%> (-3.94%)	⬇️
...gle/cloud/spanner/pgadapter/ConnectionHandler.java	79.52% <75.00%> (-0.16%)	⬇️
...ogle/cloud/spanner/pgadapter/commands/Command.java	100.00% <100.00%> (ø)
.../cloud/spanner/pgadapter/commands/ListCommand.java	100.00% <100.00%> (ø)
...panner/pgadapter/statements/BackendConnection.java	93.04% <100.00%> (+0.62%)	⬆️
...apter/statements/ExtendedQueryProtocolHandler.java	100.00% <100.00%> (ø)
...apter/statements/local/ListDatabasesStatement.java	100.00% <100.00%> (ø)
...ud/spanner/pgadapter/utils/ClientAutoDetector.java	96.66% <100.00%> (+0.37%)	⬆️

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