Merge bitcoin/bitcoin#27279: Add "warnings", deprecate "warning" in {create,load,unload,restore}wallet

7ccdd741fe test: fix importmulti/importdescriptors assertion (Jon Atack)
19d888ce40 rpc: move WALLET_FLAG_CAVEATS to the compilation unit of its caller (Jon Atack)
01df011ca2 doc: release note for wallet RPCs "warning" field deprecation (Jon Atack)
9ea8b3739a test: createwallet "warning" field deprecation test (Jon Atack)
645d7f75ac rpc: deprecate "warning" field in {create,load,unload,restore}wallet (Jon Atack)
2f4a926e95 test: add test coverage for "warnings" field in createwallet (Jon Atack)
4a1e479ca6 rpc: add "warnings" field to RPCs {create,load,unload,restore}wallet (Jon Atack)
079d8cdda8 rpc: extract wallet "warnings" fields to a util helper (Jon Atack)
f73782a903 doc: fix/improve warning helps in {create,load,unload,restore}wallet (Jon Atack)

Pull request description:

  Based on discussion and concept ACKed in #27138, add a `warnings` field to RPCs createwallet, loadwallet, unloadwallet, and restorewallet as a JSON array of strings to replace the `warning` string field in these 4 RPCs. The idea is to more gracefully handle multiple warning messages and for consistency with other wallet RPCs.  Then, deprecate the latter fields, which represent all the remaining RPC `warning` fields.

  The first commit https://github.com/bitcoin/bitcoin/pull/27279/commits/f73782a9032a462a71569e9424db9bf9eeababf3 implements https://github.com/bitcoin/bitcoin/pull/27138#issuecomment-1474789198 as an alternative to #27138. One of those two could potentially be backported to our currently supported releases.

ACKs for top commit:
  achow101:
    ACK 7ccdd741fe
  1440000bytes:
    utACK https://github.com/bitcoin/bitcoin/pull/27279/commits/7ccdd741fe1544c13b2a9b7baa5c5727e84d6e55
  vasild:
    ACK 7ccdd741fe
  pinheadmz:
    re-ACK 7ccdd741fe

Tree-SHA512: 314e0a4c41fa383d95e2817bfacf359d449e460529d235c3eb902851e2f4eacbabe646d9a5a4beabc4964cdfabf6397ed8301366a58d344a2f787f83b75e9d64

doc/release-notes-27279.md

@@ -0,0 +1,10 @@
+Wallet
+------
+
+- In the createwallet, loadwallet, unloadwallet, and restorewallet RPCs, the
+  "warning" string field is deprecated in favor of a "warnings" field that
+  returns a JSON array of strings to better handle multiple warning messages and
+  for consistency with other wallet RPCs. The "warning" field will be fully
+  removed from these RPCs in v26. It can be temporarily re-enabled during the
+  deprecation period by launching bitcoind with the configuration option
+  `-deprecatedrpc=walletwarningfield`. (#27279)