walletcreatefundedpsbt needs to (optionally) add explicit value fields

[philippem]

In a recent change to the PSET spec, new input keys were added to support the communication of unblinded values, for the benefit of resource-constrained environments. See the requesting issue #1153

The new input keys are

PSBT_ELEMENTS_IN_EXPLICIT_VALUE
PSBT_ELEMENTS_IN_VALUE_PROOF
PSBT_ELEMENTS_IN_EXPLICIT_ASSET
PSBT_ELEMENTS_IN_ASSET_PROOF
PSBT_ELEMENTS_IN_BLINDED_ISSUANCE

Support for these keys was added in the PRs

#1157
#1165

The current walletcreatefundedpsbt RPC does not set these fields in its response, nor provides a way for the caller of the RPC to request that they be populated (possibly by using the 4th optional options argument).

To reproduce

See the test vector generator provided at
https://github.com/miketlk/testvectors/blob/ca7a6a2f00a61d881168a2ae498fee05e130c1fe/tests/test_pset.py#L123-L146

System information

elementsd commit e0c911a on elements-22.x branch

[apoelstra]

cc @achow101 can you add an option to walletcreatefundedpsbt to populate these fields?