feat(dashboards): register BFCache Metrics prebuilt dashboard

[logaretm]

Adds the BFCACHE_METRICS dashboard to the FE along with an onboarding content flag.

BFCache is a browser memory cache that only works for backward/forward navigations, but the browser has certain heuristics for deciding if a page is eligible or not.

This PR builds on several other PRs around Sentry to expose the cache heuristics to the developer

• feat(attributes): Add browser.bfcache.* attributes sentry-conventions#513
• feat(browser): Add bfcacheIntegration sentry-javascript#22397

Reference links:

• https://web.dev/articles/bfcache

[github-actions]

🚨 Warning: This pull request contains Frontend and Backend changes!

It's discouraged to make changes to Sentry's Frontend and Backend in a single pull request. The Frontend and Backend are not atomically deployed. If the changes are interdependent of each other, they must be separated into two pull requests and be made forward or backwards compatible, such that the Backend or Frontend can be safely deployed independently.

Have questions? Please ask in the #discuss-dev-infra channel.

[getsantry]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀