[assetserver] Add support for serving the index.html file when requesting a directory

[stffabi]

Furthermore this PR improves some error messages for the AssetServer Options and adds some additional checks.

Fixes #2109

[mholt]

Ohh, very nice. I'll give this a try!

[stffabi]

@mholt Just wanted to ask if you eventually found time to give this a try?

[mholt]

@stffabi Sorry for the wait! 😓 I haven't been available much with the holiday and such this week. Thanksgiving is a very big day in the US that disrupts nearly the whole week. I'll try to look at this soon. (Don't wait on me though, if you want to merge it in sooner, I don't want to be a blocker.)

[stffabi]

I’m sorry, I really didn’t want to stress you.
Hope you had a wonderful time with your beloved ones.

[leaanthony]

Thanks for doing this - this is great! I'm going to open a new ticket that I'll do after this has been merged to update the default index.html page with better information on why it is being shown and how to fix it 👍

[stffabi]

Thanks for approving 🙏

[mholt]

Thank you again for doing this :) I am hoping to catch up to it this week!

Really appreciate all you both do to work on this project 🙏