feat: add headers when loading URLs, closes #816

[amrbashir]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

[lucasfernog]

Initially I thought with_headers would apply to all URLs which is not the case, so maybe we chould change the signature to with_url_and_headers.

[lucasfernog]

I've changed the signatures to use the existing methods instead of adding new ones. So now with_url, with_html and load_url takes an Option<http::HeaderMap>. Let me know what you think @amrbashir I think this is less confusing to users.

[amrbashir]

There is no point in using headers with with_html since it loads local content.
Also, most users won't use headers, so I think it should be a separate function. I like the idea of with_url_and_headers instead.

[lucasfernog]

Cool @amrbashir