feat: add default privileges to _pico_privilege for new users

Summary

grant default tarantool privileges to user after creating:

• execute on public
• session and usage on universe
• alter on himself

Close #385 (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