Add User-Agent with Puppeteer version to WebSocket request

[headlesstesting]

This will add the User-Agent header to WebSocket requests, with the Puppeteer version.

The reason why we wanted to create this PR is because we can then offer our customers the most compatible browser version depending on the Puppeteer version they are using to connect to our service.

Developers use this method to connect to our service:
const browser = await puppeteer.connect({ browserWSEndpoint: 'wss://chrome.headlesstesting.com' })

When we receive a request, we can check the user-agent and determine the most compatible browser version.

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[headlesstesting]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

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[jschfflr]

@headlesstesting This change looks good to me, thanks!

[jschfflr]

@headlesstesting Can you resolve the conflicts? Happy to get this landed!

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[google-cla]

All (the pull request submitter and all commit authors) CLAs are signed, but one or more commits were authored or co-authored by someone other than the pull request submitter.

We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that by leaving a comment that contains only @googlebot I consent. in this pull request.

Note to project maintainer: There may be cases where the author cannot leave a comment, or the comment is not properly detected as consent. In those cases, you can manually confirm consent of the commit author(s), and set the cla label to yes (if enabled on your project).

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[jschfflr]

@googlebot I consent.