feat: propose new features for the Postgres matrix

[gbartolini]

• Mutual TLS support for authentication of replicas
• JSON logging in standard output
• Audit logging
• Read-only root file system
• Transparent change of configuration for PostgreSQL parameters
• Fencing

[ahachete]

@gbartolini May you kindly review the above comment? :) It would be great to merge your improvements soon. Thanks!

[gbartolini]

@ahachete do you have any further comments? Thanks!