[#73455918] Set user agent for crawler

Set the crawler's HTTP user agent to "GOV.UK Crawler" and add a test for
this.
Should this be "GOV.UK Crawler Worker" to match the project name?

I was going to add that it would be useful to include the version number like [govuk_mirrorer does](https://github.gds/gds/govuk_mirrorer/pull/5) because we've had difficulty in the past working out what/where it was running from. But we don't currently have a version, so we should create a separate story for that.