[Feature Request] Separate downloading and parsing

[lonix1]

There seem to be a number of open issues (e.g. #55 #42 #29 #22) related to the downloading of a feed. One of them (#55) is quite important.

I think the library is excellent, but it does too much.

Parsing:
It would be nice if we could use it for parsing only, by supplying our own XML strings.

Downloading:
For those who want to use it for downloading as well, the existing methods could be used (so I'm not referring to a breaking change). However, the HttpClient should be initialised in a static constructor, so that only those users who want it will actually cause it to be created. Those who don't use it - and only use the parser - would not cause the client to be created.

[Roald87]

I forked this repo and removed all the HttpClient stuff from it. The library only does parsing now. Also I ported to .NET 8.

https://github.com/Roald87/FeedReader