Move RelationalDB to spacetimedb-engine crate.

[Shubham8287]

Description of Changes

Moves RelationalDB and related database code into a new spacetimedb-engine crate.
The main motivation is to tighten dependency control around the engine layer and isolate RelationalDB
behind a crate boundary.

• Majority of this PR is code-motion.
• Removes direct production dependence on tokio from spacetimedb-engine.
• Keeps tokio only as a dev-dependency for test-only code in spacetimedb-engine.
• This is intended to be a structural refactor only and should not result in any functional change in
  production.
• Adds a CI check to ensure spacetimedb-engine continues to compile in simulation mode

API and ABI breaking changes

NA

Expected complexity level and risk

Testing

Existing tests should be enough.

[cloutiertyler]

Why is "Expected complexity level and risk" unspecified? Could you please update. I assume you're saying it's low risk because it's just mechanical?

[Shubham8287]

Why is "Expected complexity level and risk" unspecified? Could you please update. I assume you're saying it's low risk because it's just mechanical?.

No, I would have come to back it, when asked someone to review it. It is draft rn. (I should have marked).

[Shubham8287]

All the crates, which are directly or in-directly depend on "runtime" crate, now have to do this "feature forwarding".

To be able to compiled with simulated runtime when "simulated" feature is on.

Not a fan of doing it this way, but not sure about better ways.