Add command and HTTP API to hash a value according to our algorithm

[lukebakken]

https://www.rabbitmq.com/passwords.html#computing-password-hash

https://groups.google.com/g/rabbitmq-users/c/3EXvhb8uLnU

https://stackoverflow.com/questions/41306350/how-to-generate-password-hash-for-rabbitmq-management-http-api

Since passwords must be hashed when used in a definitions file we should provide a canonical way or ways of doing it.

[lukebakken]

Tangentially related to #3350