[SERVE] Allow adjustment of scaling policies without redeployment

[JGSweets]

Currently, when altering the replica_policy, update runs a pseudo blue-green deployment in the sense it launches all new resources.

Preferably, if only the replica_policy is changing, it alters the policy itself without deploying /tearing down new instances unless required by the new policy.

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Example 1:
Init: Currently, 2 resources are running, but the min_replica is set to 3.
Result: Only 1 instance is launched.

Example 2:
Init: Currently, 2 resources are running, but the min_replica is set to 1 and qps would not be met if scaled down.
Result: 1 instance is torn down.

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Solution Options:

1. Have update check to make sure only the replica_info has changed with a hash.
2. Use a flag with update that allows altering just the replica policy
3. Have a separate update endpoint which allows updating the replica policy.

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Version & Commit info:
skypilot, version 0.7.0
skypilot, commit 3f62588

[Michaelvll]

cc'ing @cblmemo