feat(webapp,plugins): provision a metering customer when an organization is created

Adds an opt-in billing plugin, in the same shape as the SSO and RBAC
plugins: a contract in `@trigger.dev/plugins`, a loader in
`@trigger.dev/billing` that dynamically imports an installed
implementation and falls back to a no-op, and the host wiring in the
webapp.
Mono-RevId: d69c1b3adab90b77f66cf5930ea3516e48a3fe6a