[Serve] Add policy config to AutoscalingConfig

[GeneDer]

Why are these changes needed?

Move BasicAutoscalingPolicy to a public file and add the new policy config to AutoscalingConfig.

Related issue number

First PR for #41135

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  • Unit tests
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  • This PR is not tested :(

[GeneDer]

@edoakes would be nice if you can give this a look as well

[shrekris-anyscale]

Nice work! I left some comments.

[edoakes]

Mostly concerned about this base64 encoding and dynamic behavior of the policy field.

If something nonstandard like this is required in a PR I expect it to be called out in the description to let reviewers know why it's necessary and what alternatives there are, if any.

[GeneDer]

@edoakes @shrekris-anyscale I have addressed all the comments and removed the need to serialize policy with base64 (thanks to the idea on separating them into 2 fields). PTAL again when you have a sec!