[DPE-7726] Use Patroni API for is_restart_pending() (instead of SQL select from pg_settings)

[taurus-forever]

Issue

The previous is_restart_pending() waited for 15 seconds due to the
Patroni's loop_wait default value (10 seconds), which tells how much time
Patroni will wait before checking the configuration file again to reload it.

Solution

Instead of checking PostgreSQL pending_restart from pg_settings,
check Patroni API pending_restart=True/undefined.

Checklist

• I have added or updated any relevant documentation.
• I have cleaned any remaining cloud resources from my accounts.

[codecov]

Codecov Report

❌ Patch coverage is 29.54545% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.53%. Comparing base (823e0a0) to head (e31de74).

Files with missing lines	Patch %	Lines
src/charm.py	30.76%	16 Missing and 2 partials ⚠️
src/cluster.py	23.52%	13 Missing ⚠️

❌ Your patch check has failed because the patch coverage (29.54%) is below the target coverage (33.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project check has failed because the head coverage (64.53%) is below the target coverage (70.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##           16/edge    #1049      +/-   ##
===========================================
- Coverage    65.84%   64.53%   -1.31%     
===========================================
  Files           17       17              
  Lines         4178     4182       +4     
  Branches       609      593      -16     
===========================================
- Hits          2751     2699      -52     
- Misses        1284     1351      +67     
+ Partials       143      132      -11     

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