Add Drain mode to PaneInfo

[kennknowles]

This is not yet instantiated by any runner, so will always be the default value, and never included in encoded PaneInfo since default values do not result in extended metadata being added.

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[kennknowles]

Now that I have gotten this far into implementation, I can see that the SDK does not need this value. It is necessary in protobuf but should be converted to NOT_DRAINING because we want to never encode it unless it is draining.

[github-actions]

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[kennknowles]

Closing this in favor of new suggestion by @stankiewicz