Support runtime refresh of the DB username (not just password) for rotating dynamic credentials

[Roxyrob]

Is your feature request related to a problem?

Temporal can refresh the DB password at runtime (cfg.PasswordCommand → a refreshable DSN),
but the username is fixed in static config. Credential systems that rotate both fields
per lease — HashiCorp Vault's database secrets engine (per-lease users like entity:serv-xxxx) —
therefore cannot be used without a pod restart on every rotation. This is the sibling of #9156:
both need credentials re-resolved at runtime, but #9156 rotates a token (password-like) while
this rotates the username too.

What already exists (main)

• common/persistence/sql/sqlplugin/db_handle.go — DatabaseHandle.connect func() (*sqlx.DB, error)
  is re-invoked on every reconnect(force); ConvertError triggers reconnect(true) on
  needsRefresh(err) and on connection errors (ErrBadConn, EOF,
  ECONNRESET/ECONNABORTED/ECONNREFUSED). Combined with MaxConnLifetime, connections are
  periodically rebuilt and the connect callback re-runs.
• common/persistence/sql/sqlplugin/postgresql/session/session.go — createConnection already
  supports a refreshable DSN via cfg.PasswordCommand → CreateRefreshableConnection(buildDSN).

The gap

buildDSN reads cfg.User as a static value, so even the refreshable path only refreshes the
password. There is no equivalent of PasswordCommand for the username.

Proposed solution

Extend the refreshable-credentials mechanism so the username is re-read on each connect() —
e.g. a UserCommand (analogous to PasswordCommand) or a credentials-provider that yields both
username and password at DSN-build time. On reconnect (driven by MaxConnLifetime or
ConvertError), the pool then re-authenticates with the rotated username+password without a
restart. This reuses the existing DatabaseHandle.reconnect() + PasswordCommand infrastructure,
generalizing the password refresh already present.

Environment

Version: v1.31.2 (mechanism unchanged on main). Plugin: postgres12 / postgres12_pgx.
Credentials: Vault per-lease username+password delivered as a K8s Secret by the Vault Secrets
Operator.

Related

• Cloud Database Cert/IAM Connections Cloud Database Cert/IAM Connections #9156 (cloud/IAM DB auth — same runtime
  credential-resolution need; notes DatabaseHandle.reconnect() already re-runs the connect
  callback).
• DSN username-encoding bug (buildDSN interpolates cfg.User raw, breaking usernames with :)
  — tracked separately.
• Chart-side prerequisite (sourcing the username from a Secret): Allow DB username from a Secret (like password) to support rotating dynamic credentials (Vault) helm-charts#985.