Add support for the ALTER USER statement to change account passwords

[fulghum]

Adds basic support for the ALTER USER statement, so that users can change passwords. All users are allowed to change their own password; changing another user's password requires the CREATE USER privilege or the UPDATE privilege on the mysql database.

MySQL reference docs for ALTER USER

Related to: dolthub/dolt#7348

[max-hoffman]

lgtm, I'm not too familiar with the auth execution code but seems reasonable to me