VAULT-15840: Allow updates of only entity-alias custom-metadata

[miagilepner]

With these changes, the following works. Previously, the final call would return an empty map:

$ vault auth enable userpass 
$ MA1=$(vault auth list -format=json | jq -r '."userpass/" | .accessor')
$ EID=$(vault write -field=id identity/entity name=max-winslow policies=entity-policy)
$ AID=$(vault write -field=id identity/entity-alias name=max1 canonical_id=$EID mount_accessor=$MA1)
$ vault write identity/entity-alias/id/$AID \
   - <<<'{"custom_metadata":{"foo":"bar"}}'
$ vault read -field=custom_metadata identity/entity-alias/id/$AID
map[foo:bar]

Closes #19434