Add copy actions to network requests (Copy as URL, cURL)

[jankuss]

This PR adds the ability to copy HTTP requests in the Network screen.
Right now it is possible to Copy the URL, and to Copy as a cURL command.

Screen.Recording.2022-05-18.at.00.25.24.mov

This closes #3042

As per #3042 (comment), we use the Icons.more_vert for the button which opens the menu.
The menu button is only shown when the request is selected.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read the [Flutter Style Guide] recently, and have followed its advice.
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[kenzieschmoll]

some initial comments. Thanks for your work on this!

[jankuss]

Thanks for the review 👍

I have made adjustments based on your suggestions. I think the CurlCommand class is now easier to grasp.

The comments #4113 (comment) and #4113 (comment) clashed a little, since the _headers() and _body() couldn't be called in the factory because they were instance members. I solved this by marking them static, and passing the required information as parameters instead.

In this case when we reduce this even further, the properties DartIOHttpRequestData data, bool followRedirects and bool multiline are not needed anymore, as they are not accessed in any way. The only thing that remains is the List<String> commandParts.

[kenzieschmoll]

LGTM. Thanks for your contribution of this feature!