feat: populate builtin users and roles with their permissions

Summary

following builtin roles and users with their privileges persisted in bootstrap snapshot:

• role super with full privileges
• role public with no privileges
• user guest with role public, super and privilege session, usage
• user admin with full privileges

Close #384 (closed)

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• (if Lua API changed) Lua API version is bumped in luamod.rs
• (if necessary) A follow-up doc issue is created in picodata/docs and linked here