feat: make postgres SSL certificate paths configurable

[blink-so]

This PR fixes the hardcoded certificate paths introduced in PR #40 by making the SSL certificate configuration fully customizable.

Changes

• Removed hardcoded paths: No more /home/coder/.postgresql defaults
• Made sslrootcert configurable: Now accepts custom certificate paths or can be left empty
• Made volumes and volumeMounts configurable: Users can specify their own volume configuration
• Added conditional rendering: Prevents empty volume mounts from being rendered
• Updated connection string logic: Handles optional sslrootcert parameter gracefully
• Updated documentation: README reflects new default values

Configuration Example

Users can now configure SSL certificates like this:

global:
  postgres:
    sslmode: require
    sslrootcert: /custom/path/to/rootcert.pem
    volumes:
      - name: "custom-certs"
        configMap:
          name: "my-postgres-certs"
    volumeMounts:
      - name: "custom-certs"
        mountPath: "/custom/path/to"
        readOnly: true

Breaking Change

This is a breaking change for users who relied on the hardcoded defaults. Users will need to explicitly configure SSL certificate paths if they use SSL connections.

Fixes the issue where certificate paths were hardcoded to /home/coder/.postgresql/.