[ENHANCEMENT] Remove Local Storage Requirement When Using an External Database

[ElliotAtKlaviyo]

If one configures an external database, Phoenix still requires a working directory. In other words, setting PHOENIX_SQL_DATABASE_URL also requires PHOENIX_WORKING_DIR, which might be unexpected for users. If possible, the external database should store all data.

See this Slack thread for additional context. @mikeldking explained that "The boot time of phoenix does require it to have write access to [a working] directory but it will largely go un-used when you use sql". @RogerHYang pointed to this constraint in the code.