[hkarmoush]

Part of #8190

The Headers and Cookies tabs in the network request inspector had no way to copy their contents, and the cookie table and image metadata weren't selectable at all. This adds a copy button to the tab bar for Headers and Cookies, the same as Request and Response already have, so all the tabs work the same way.

• Headers copies each section (General, Response Headers, Request Headers) under its title as key: value lines. Multi-value headers are joined the way they'd be sent (set-cookie gets one line per value, cookie is joined with ; ), so the text can be pasted straight into curl or a bug report.
• Cookies copies response cookies as their raw Set-Cookie values (parsing drops Max-Age and SameSite, so the raw header is used when it's there) and request cookies as a Cookie header value.
• Cookie cells and image metadata are now inside a SelectionArea. Selection doesn't work in VS Code on Mac (SelectableText and SelectionArea do not work correctly when calling document.execCommand("copy") unless you right-click flutter#182756), which is why the copy buttons are the main fix here.

While in there I also fixed calculateHeadersSize, used for the HAR export. It checked value is List<String>, but headers decoded from JSON are List<dynamic>, so list values were counted with their brackets. It now shares the same formatting as the copy text.

Per-row copy and the request list actions menu mentioned in the issue are left for a follow-up.

Before/after screenshots to come.

Pre-launch Checklist

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