feat: add postgres.noPasswordEnv

[johnstcn]

This PR adds a Helm flag postgres.noPasswordEnv, defaulting to false. If enabled:

• The environment variable DB_PASSWORD is no longer set to the value of the key password of the Kubernetes secret named postgres.passwordSecret for containers in the coderd deployment.
• The Kubernetes secret named postgres.passwordSecret is mounted under /run/secrets/{{ postgres.passwordSecret }} (~0400~ originally mounted 0400 but coderd is unable to read this)
• The environment variable DB_PASSWORD_PATH is set to /run/secrets/{{ postgres.passwordSecret }}/password for containers in the coderd deployment.

Verification steps:

• [x] Validated that Helm deploy with defaults does not change behaviour
• [x] Validated that setting postgres.noPasswordEnv in Helm values causes Coder to read the postgresql password from the file specified by DB_PASSWORD_PATH.