feat: support gcp secret manager for root user credentials

[nakamasato]

Why

For better credential management

What

• Enable to read admin password from GCP SecretManager
• Bump controller-gen to v0.9.0 to fix error & update Makefile

Errors

1. panic: interface conversion: ast.Expr is *ast.CompositeLit, not *ast.Ident

1. https://github.com/nakamasato/mysql-operator/actions/runs/4601260750/jobs/8128914277

   /home/runner/work/mysql-operator/mysql-operator/bin/controller-gen object:headerFile="hack/boilerplate.go.txt" paths="./..."
   panic: interface conversion: ast.Expr is *ast.CompositeLit, not *ast.Ident
   

Solution: bump controller-gen to v0.9.0 that includes fix kubernetes-sigs/controller-tools#648

Updated Makefile following https://github.com/kubernetes-sigs/kubebuilder/blob/1f099eaf68d95df2edcb1d0f88a21af356bf279a/docs/book/src/migration/legacy/manually_migration_guide_v2_v3.md

Check list

• run in local
• run with service account
• test
  • update/delete MySQLClients when adding MySQL object
• update diagram
• add diagram to CONTRIBUTING.md

[codecov]

Codecov Report

Merging #190 (678173c) into main (0d6bb49) will decrease coverage by 1.66%.
The diff coverage is 42.55%.

@@            Coverage Diff             @@
##             main     #190      +/-   ##
==========================================
- Coverage   65.51%   63.86%   -1.66%     
==========================================
  Files           6        6              
  Lines         522      559      +37     
==========================================
+ Hits          342      357      +15     
- Misses        139      156      +17     
- Partials       41       46       +5     

Impacted Files	Coverage Δ
controllers/mysql_controller.go	52.31% <42.55%> (-3.83%)	⬇️

... and 2 files with indirect coverage changes

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