Use spacetimedb-standalone to get schema for spacetime generate

[coolreader18]

Description of Changes

In preparation for adding another host type, I realized it would make sense to have it so we don't have to compile huge runtimes for spacetimedb-cli in addition to spacetimedb-standalone. Now, spacetime generate shells out to spacetimedb-standalone, which uses host_controller.check_module_validity() to load the module and get the ModuleDef.

Expected complexity level and risk

2: Introducing another way that the binaries interact with each other, but this is altogether pretty small and straightforward.

Testing

• [x] Ensured spacetime generate still works.
• [ ]

[coolreader18]

Apologies for the rebase; there was a big conflict with #2593 and it didn't really make sense to do a merge.

[CLAassistant]

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