Allow specifying custom PowerShell config and cache directory instead of the hardcoded paths in `Platform`

Summary of the new feature/enhancement

Currently, it is possible to download PowerShell Core as a .zip archive and unzip it to a custom directory and everything seems to work correctly even from non-standard path, so users could run pwsh as a portable package by putting it e.g. on a USB flash drive and just plugging it in whenever needed.

However, the config directory (where profile and installed modules are stored; at Documents/Powershell) and cache directory (at LocalAppData\Microsoft\PowerShell) paths are currently hardcoded. It would be great if we could specify custom paths in powershell.config.json or another similar config file, ideally with support for paths relative to the pwsh.exe binary.

Proposed technical implementation details (optional)

It seems that both directories are configured here: https://github.com/PowerShell/PowerShell/blob/1a3721a574e00263f64101d5dbbe1a975e6c8087/src/System.Management.Automation/CoreCLR/CorePsPlatform.cs#L149

As far as I can tell, it should be enough to check if there are any paths configured in the config file and set them here (and ideally abort with an error if the paths do not exist). I don't have much experience with C#, but I can try writing a pull request if we agree on a solution.

Start with src/System.Management.Automation/CoreCLR/CorePsPlatform.cs around the path definitions referenced in the issue, then trace how powershell.config.json is loaded. Determine how custom config and cache paths, including paths relative to pwsh.exe, should be represented and validated. Done means portable installations can use configured directories instead of the current hardcoded paths.