Opens the 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
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.