Change Redpanda service account password

[jrkinley]

Console should provide the ability to change a user's password, without having to delete and recreate the user and their associated ACLs.

[twmb]

@ivpanda for mocks

[ivpanda]

Just commenting to close the loop; this seems low priority, so I haven't done any mocks for it yet. As soon as it's prioritized product-wise, I'll jump on it.