Add ResourceDrift and ProposedUnknown fields to Plan

[dcolmenares-r7]

Adding fields resource_drift and proposed_unknown outlined in https://developer.hashicorp.com/terraform/internals/json-format#plan-representation.

This change will allow developers to better use this library and access all relevant fields of the plan.

[hashicorp-cla]

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[gdavison]

@dcolmenares-r7, make sure that you sign the Contributor License Agreement for your PR. Otherwise it won't be merged

[dcolmenares-r7]

@gdavison Is there a corporate CLA that Rapid7 can sign instead of individual - I've reached out to our legal and I don't think I'm able to sign an individual CLA. Anyone I can follow up with this about?