feat: add fields in SinglePlatformSpec

[chenyifan-vertex]

Description of your changes:
Add two fields in SinglePlatformSpec: platform and config.
This will be used by Platform Specifc features and currently used by Vertex Preflight validations.

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[chensun]

/lgtm
/approve

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[gregsheremeta]

@chensun @chenyifan-vertex consider this my gentle protest about merging a PR 8 minutes after it was posted 😂

@chensun are these new fields something we can/should use for the (non-Vertex) kubernetes platform? If so, how? I'm specifically thinking about PipelineConfig, which we were going to add as a sibling to deployment_spec. But now I see there is a Struct config, and I wonder if it makes more sense to nest PipelineConfig under that.

Perhaps some examples of how you're using config in Vertex would help.

Or if you think PipelineConfig should just be PipelineConfig pipelineConfig = 4 sibling of deployment_spec, platform, and config, that's fine too.