Merge #14291: wallet: Add ListWalletDir utility function

Summary:
d56a0689354fb814510c6c393f3e07ac9362dc1f docs: Add release notes for listwalletdir RPC (João Barbosa)
0cb3cad166bbeb75e9cc1512286453f8e7d4f717 qa: Add tests for listwalletdir RPC (João Barbosa)
cc3377360c417780f5cbd7bd69b438817a9d60be rpc: Add listwalletdir RPC (João Barbosa)
d1b03b8e5f04a2cc9ebb985bd9a1aebd2068f757 interfaces: Add getWalletDir and listWalletDir to Node (João Barbosa)
fc4db35bfd78d85d6b52d5da3d89696160658450 wallet: Add ListWalletDir utility (João Barbosa)

Pull request description:

`ListWalletDir` returns all available wallets in the current wallet directory.

Based on MeshCollider work in pull #11485.

Tree-SHA512: 5843e3dbd1e0449f55bb8ea7c241a536078ff6ffcaad88ce5fcf8963971d48c78600fbc4f44919523b8a92329d5d8a5f567a3e0ccb0270fdd27366e19603a716

Backport of Core PR14291

Test Plan:

ninja
ninja check
ninja check-functional
./bitcoind
./bitcoin-cli help

Verify listwalletdir appears in the list of Wallet RPCs.

./bitcoin-cli help listwalletdir
./bitcoin-cli listwalletdir

Verify new RPC help text and behavior.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5527