UrlStorage blacklist of forwarded headers

[KrKOo]

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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?

[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! 🎉