UrlStorage blacklist of forwarded headers

[KrKOo]

This PR introduces the ability to define headers that should not be forwarded when forward_headers is enabled, as described in issue #243.

Example config

...
[resolvers.storage]
url = "http://localhost:8080"
response_url = "https://example.com"
forward_headers = true
header_blacklist = ["Host"]
...

[brainstorm]

This is great, thanks @KrKOo! I'm missing some documentation though, could you please describe your example in the docs and provide the resolver config you mentioned in the description of this PR?

Thanks again!

@mmalenic Any other observations? Does this PR fit with your recent hyper->reqwest transition?

[mmalenic]

Thanks a lot for this @KrKOo. This is really good, and thanks for testing. Other than some documentation like @brainstorm said, could you please also run cargo fmt, this should fix the CI tests.

@mmalenic Any other observations? Does this PR fit with your recent hyper->reqwest transition?

All good in relation to hyper/reqwest, these changes aren't affecting that portion of the code as they only edit the out-going headers in the URL tickets.

[KrKOo]

Hey, while writing the docs, I realized that the headers were not removed from the request made by htsget-rs to the resource server, so I added that as well.

[mmalenic]

This is great, thanks @KrKOo!

[brainstorm]

Thanks again @KrKOo for your contribution! 🎉