Opens the non-free-editable fields of a Proposal for edit.
This proposal will not receive updates from Marketplace while
it's open for edit. If the buyer updates the proposal while it is
open for local editing, Google will set ProposalMarketplaceInfo.isNewVersionFromBuyer to true. You
will then need to call DiscardProposalDrafts to revert your
edits to get the buyer's latest changes, and then perform this
action to start making your edits again.
This action is only applicable for programmatic proposals.