[WIP] psbt explicit optional fields

psbt-optional-fields.md

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+## Abstract
+
+In general, if encountering PSBT fields that are unrecognized, to avoid costly
+mistakes it is best to fail and not assume that those fields are backwards
+compatible.
+
+However, some information, for example BIP 353 `PSBT_OUT_DNSSEC_PROOF`, and may
+safely be ignored by devices that in this case might not able verify the BIP
+353 DNSSEC signature chain.
+
+## Specification
+
+Top level `PSBT_GLOBAL_RULESETS`. keydata contains identifier e.g. `BIP <n>`
+for well known protocols or arbitrary domain separators (encoded like
+proprietary keys or or as hashes). rationale: this allows definition of machine
+readable schema of rules that are supposed to be enforced, something which may
+not be self evident from just the fields depending on the state of the PSBT.
+
+Top level `PSBT_GLOBAL_OPTIONAL_FIELDS` - keydata identifying the fields that
+are safe to ignore. these are presumed to correspond to the active rulesets.
+
+---
+
+active BIPs or domain separators could refer script related ones, which
+suggests the possibility of adding global policy scripts (as keydata to a
+single global field) that need to validate before signing, with the value
+optionally being used to specify evaluation rules or provide witness data. For
+example if simplicity is used to specify a zk proof validator, then signers
+should only sign if this evaluates to true. signaling such policy rules in a
+consistent manner would allow relatively safe interoperability even for
+conceivably quite complicated scripts, and facilitates experimentation by
+providing a generic procedure for multisig covenant emulation by all input
+signers sharing a PSBT.