source-postgres: Feature flag use_schema_inference

[willdonnelly]

Description:

This PR adds a feature flag use_schema_inference to source-postgres which when set causes x-infer-schema: true to be added to discovered schemas.

Actually figuring out the right spot to add the x-infer-schema annotation was surprisingly tricky, what I ended up doing was adding a third "partial schema" to the top-level allOf, where the new schema is literally just {"x-infer-schema": true}. I think that works?

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This change is 

[jgraettinger]

This actually doesn't work: our handling of x-infer-schema is simplistic and we only inspect it if it's a property of the schema root. Would it be easier to instead unconditionally include x-infer-schema, but vary whether it's value is true or false ?

[willdonnelly]

This actually doesn't work: our handling of x-infer-schema is simplistic and we only inspect it if it's a property of the schema root

Ah, I see. Okay, I've uploaded a new implementation then, which I think puts it in the right spot. The generated schema will now look like:

{
    "$defs": {"...definition of the actual table properties..."},
    "allOf": [
        {"...the usual stuff..."},
        {"...and the reference to the main table properties definition..."}
    ],
    "x-infer-schema": true
}

Does that seem correct to you? I opted to still only add it conditionally because it was about as easy to do it either way and I prefer knowing that the default schema generation is 100% unaltered by this PR.

[jgraettinger]

LGTM