added storage dir path options

[Smug246]

From discussion #1418

[rodja]

I think we should not use the temp directory as default to store persistent data. temp gets potentially cleaned a lot, and would defeat the purpose of the persistence.

[falkoschindler]

Thanks for the quick PR, @Smug246! Just a few remarks:

• I have to agree with @rodja. At least in 1.3 we don't want to introduce breaking changes. So the default path should be the current working directory.
• And I slightly tend to the parameter name storage_path instead of storage_dir to avoid an abbreviation and to indicate its path-like nature.
• Last but not least we shouldn't forget to add this parameter to ui.run_with for when using NiceGUI with an existing FastAPI app.

[Smug246]

np ill make those changes

[falkoschindler]

Thanks, @Smug246! I just made a few more changes:

• I removed the special case of "temp". It doesn't make much sense to store data in the temp directory which might not survive a reboot. And it could be confused with a folder named "temp" in the current working directory. Therefore I removed it altogether. This doesn't prevent the user from explicitly setting the temp directory.
• I converted the value to a Path object before storing it in globals. This way we can use it more easily.
• I set the default value to Path('.nicegui') instead of None. This communicates the default value more clearly, removes the need for Optional and avoids the need for handling None later.

A downside of these changes is the duplication of the default value Path('.nicegui'). But since the parameter list of ui.run and ui.run_with is duplicated anyway, I guess this is a reasonable tradeoff.

[falkoschindler]

Oh dear, I merged too early. The storage path is already used before ui.run is called. Therefore the app can't start.
I'll somehow have to undo this PR on main...

[falkoschindler]

This problem seems to be pretty fundamental. Therefore I reverted this PR and re-opened discussion #1418.