feat: Add global stack event registry and block requests before ready

[msfstef]

PR by @icehaunter and me - makes the StackSupervisor accept a stack event registry that it uses to dispatch status events about the state of the stack.

This was preliminary work for multitenancy, and also fixes https://github.com/electric-sql/electric/issues/1922 since now we hold connections when the stack is not ready, and release them when we receive a "ready event" or time them out with a 503 - avoids crashing the ETS inspector which was trying to use a DB connection from an uninitialised pool.

Integration test is broken from https://github.com/electric-sql/electric/pull/2009

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