feat(postgresql): add standalone PgBouncer component

[ldming]

What

• Add replication-direct and replication-pgbouncer topologies while preserving the legacy replication topology.
• Run PgBouncer as an independent stateless component and Service on port 6432; keep PostgreSQL direct access on port 5432.
• Create the PgBouncer auth user and SECURITY DEFINER auth function only for the pooler backend.
• Validate the auth_query path with a separate low-privilege pgbouncerprobe account.
• Productize session pooling parameters and image digest and pull-policy controls.
• Advance the PostgreSQL Addon chart to 1.2.0-alpha.4.

Verification

• Targeted ShellSpec: 25 examples, 0 failures.
• Helm lint and default and digest rendering contracts pass.
• ShellCheck at the repository error severity and bash syntax checks pass.
• CUE validation and git diff checks pass.
• Independent review closed the ShellSpec contract and auth_query readiness findings.

Compatibility

• Existing replication clusters retain the historical in-pod PgBouncer sidecar.
• New product integrations select replication-direct or replication-pgbouncer.
• The first supported pool mode is session.

Known limits

• Client-to-pooler and pooler-to-PostgreSQL TLS are outside this MVP.
• Online enable or disable is outside this MVP.
• KubeBlocks system-account password rotation does not yet trigger automatic PgBouncer reload or rollout.
• The current default image remains apecloud/pgbouncer:1.19.0; a newer scanned image and pinned release digest are still required before GA.
• Real KubeBlocks 1.2 runtime, failover, backup and restore, and password-rotation validation are not included in this PR.
• APIServer and Console work is intentionally excluded and will start only after this Addon PR is merged.

Related to apecloud/apecloud#20276

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (ce67f5a) to head (9ee9c1b).

Files with missing lines	Patch %	Lines
...tgresql/scripts-ut-spec/pgbouncer_authfile_spec.sh	0.00%	18 Missing ⚠️
...postgresql/scripts-ut-spec/kb_api_contract_spec.sh	0.00%	8 Missing ⚠️
...ostgresql/scripts-ut-spec/metrics_contract_spec.sh	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #3429   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files        192     192           
  Lines      27227   27128   -99     
=====================================
+ Misses     27227   27128   -99     

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