[core] implement our own CaseInsensitiveDict

[iscai-msft]

Right now, our current implementation relies on either having the requests or aiohttp library installed. Instead, we want to implement our own case insensitive dict, so we can get rid of this dependency

[kashifkhan]

We can close this one because of #25074 right ?

[iscai-msft]

yep, closing now