[rpc] Return fee and vsize from testmempoolaccept

Return fee and vsize if tx would pass ATMP.

doc/release-notes.md

@@ -102,6 +102,9 @@ will trigger BIP 125 (replace-by-fee) opt-in. (#11413)
   option `-deprecatedrpc=banscore` is used. The `banscore` field will be fully
   removed in the next major release. (#19469)
 
+- The `testmempoolaccept` RPC returns `vsize` and a `fee` object with the `base` fee
+  if the transaction passes validation. (#19940)
+
 - The `walletcreatefundedpsbt` RPC call will now fail with
   `Insufficient funds` when inputs are manually selected but are not enough to cover
   the outputs and fee. Additional inputs can automatically be added through the