Enable cross platform builds

[guineveresaenger]

This schematizes the platform build field, which provides
cross-platform build abilities akin to the docker CLI's --platform
flag.
When set, the provider will build an image for the platform specified.

Fixes #470
Fixes #296

• Add Platform build input
• Generate SDKs

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[tlinhart]

@guineveresaenger If I see correctly, the platform field supports a string. But the --platform flag for build supports specifying multiple values for a platform. Will this be addressed?

[AaronFriel]

@tlinhart It looks like BuildKit only accepts a string.

@guineveresaenger Should we add a test here to ensure that Platform: "linux/amd64,linux/arm64" works?

[iwahbe]

Another design option would be to change platform: string to platforms: array(string). We would then natively support Platform: ["linux/amd64", "linux/arm64"]. I'm not sure if this is worth it, but its some thing we should consider.

[AaronFriel]

I prefer to hew close to what BuildKit's API & Docker CLI accept whenever possible, especially when they both agree. If we're idiomatic with those, it's easy to translate examples into Pulumi, it sets our users up for success.