feat(runtime): allow adding custom extensions to snapshot

[mlafeldt]

This makes create_runtime_snapshot more useful for embedders who add their own extension(s) to the runtime in build scripts.

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I guess the alternative would be to use the method as-is right now and then create another snapshot from that one via startup_snapshot of CreateSnapshotOptions?

[mlafeldt]

Also open to adding create_custom_runtime_snapshot or create_runtime_snapshot_with_extensions or similar instead.

[mmastrac]

Looks reasonable enough. Note that without tests this might be brittle and break in the future, however it's not an easy thing to add a test for.