Set subscription pause behavior constants

[yaroslawww]

For convenience and consistency, it is useful to have constants for pause behavior.

[CLAassistant]

All committers have signed the CLA.

[richardm-stripe]

Hello @yaroslawww. We can't merge this PR right now, as our code-generator only describes types that appear top-level resources Subscription.* and this enum appears two levels down Subscription.pause_collection.behavior. We'd like to support this eventually, but would likely do this as a library-wide effort rather than adding individual enums.

Would you be willing to re-open this as a "feature request"-labeled issue vs. a PR?