Add 'Download' button to Performance Bar

What does this MR do?

Adds a new Download button in the Performance Bar, allowing admins to easily access its raw JSON. The downloaded file will be named after the first added to the perf bar as this is typically the most recognizable url.

The truncatedUrl method is moved out of the request selector and into a static method on the performance bar store. This allows us to create the truncated url when the request is received so it can be accessed easily from both request_selector.vue and in performance_bar_app.vue.

Screenshots

perf_bar_download-720

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Works in Chrome, FF, and Safari. IE or Edge unavailable on my machine.

Closes #28222 (closed)

/cc @smcgivern, @lbot