feat(policy): add the TAIP-7 for field to Policy

[malikfaik]

Policy carries from, fromAgent and fromRole, but not for — so a policy built with this library cannot say which party it applies to, and the field is dropped when an inbound message is parsed and re-serialised.

TAIP-7 uses for alongside from on all three policy types, and implementations read it: one gateway we interoperate with feeds policy.for to its presentation request as the DID to present to, so a policy that omits it arrives with nobody to answer.

This adds the field next to from, with omitempty to match the rest of the struct, plus a round-trip test mirroring TestPolicy_FromFieldRoundTrip. go test ./... and go vet ./... pass.