Support retrieving available SKUs via /applications/{application.id}/skus

[rainbowdashlabs]

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Closes Issue: #2827

Description

Adds the ability to retrieve available SKUs via the /applications/{application.id}/skus endpoint.
Adds mapping for SKU as an extension for the SkuSnowflake

[MinnDevelopment]

I would like to wait for the open api spec to include this endpoint before merging.

See discord/discord-api-spec#37 (comment)

[MinnDevelopment]

Looks good, thanks for the contribution :)

You need to add a few missing annotations to pass the compliance test, though.

[rainbowdashlabs]

I would like to wait for the open api spec to include this endpoint before merging.

See discord/discord-api-spec#37 (comment)

Looking at the issue, is there any hope this will happen in the near future? It's open for over a year and there is no clear plan from discords site to add the openapi spec. I think that's a rather important endpoint for users that want to monetize their apps. Since discord has documentation for it, simply pushing this out might be the better option.