[DPE-5533] Add max_locks_per_transaction config option

[marceloneppel]

Issue

The max_locks_per_transaction PostgreSQL setting is not available for users to set.

Solution

Create a config option to expose it to the users.

Port of canonical/postgresql-k8s-operator#804.

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 71.82%. Comparing base (20d4597) to head (ca3bafd).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/config.py	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #718      +/-   ##
==========================================
- Coverage   71.82%   71.82%   -0.01%     
==========================================
  Files          13       13              
  Lines        3219     3226       +7     
  Branches      477      478       +1     
==========================================
+ Hits         2312     2317       +5     
- Misses        791      792       +1     
- Partials      116      117       +1     

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