Invoke SDK generation via `pulumi package gen-sdk --language ${LANG}`

[iwahbe]

Hello!

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Issue details

This provides a multitude of benefits:

• We can pin each language version independently, allowing better scoped reverts for problems like https://github.com/pulumi/pulumi/issues/14418.
• We remove a major part of our compile-time dependency on pulumi/pkg. This gets us closer to fixing https://github.com/pulumi/pulumi-vault/issues/197.

Affected area/feature

Currently, we generate a pulumi schema in memory for each language, and then invoke that languages generate function. We will need to switch to generating a pulumi schema (still per language), then pipe that into pulumi package gen-sdk.

As part of implementing this, we need to be careful to ensure that overlays continue to work as previous.

[t0yv0]

This is great. There's a bit more work to think through here involved in rolling it out, in particular working with ci-mgmt and pulumi/pulumi pinning, and making sure that upgrades work. This is not a one-and-done ticket.