Disk location
## What
Where will Cuprate store files on disk?

Closes https://github.com/monero-project/monero/pull/8202 in spirit ❤️

## Most likely
OS standards are settled and are available in the widely used https://docs.rs/dirs.

Reference:
- [XDG base directory](https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html) and the [XDG user directory](https://www.freedesktop.org/wiki/Software/xdg-user-dirs/) specifications on Linux
- [Known Folder](https://msdn.microsoft.com/en-us/library/windows/desktop/bb776911(v=vs.85).aspx) system on Windows
- [Standard Directories](https://developer.apple.com/library/content/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/FileSystemOverview/FileSystemOverview.html#//apple_ref/doc/uid/TP40010672-CH2-SW6) on macOS
- [List of XDG supporting software](https://wiki.archlinux.org/title/XDG_Base_Directory#Supported)

Examples:
| OS      | PATH                                                            |
|---------|-----------------------------------------------------------------|
| Windows | `C:\Users\Alice\AppData\Roaming\Cuprate\cuprate.toml`           |
| macOS   | `/Users/Alice/Library/Application Support/Cuprate/cuprate.toml` |
| Linux   | `/home/alice/.config/cuprate/cuprate.toml`                      |

## What else
There are some remaining questions:
- on macOS, the XDG standard could also be considered, leading to `~/.config/cuprate/cuprate.toml` there as well
- should the heavy separation of files (`~/.config`, `~/.cache`, `~/.local/share`) be skipped?
- should global files be supported (`/etc/cuprate.toml`)?

Supporting user override PATHs will also need to be supported, for all file types (config, cache, database, etc).