Wire enhanced Grafana Dashboards into RPK

[tmgstevens]

Who is this for and what problem do they have today?

Customer Success has developed a set of improved Grafana dashboards here: https://github.com/redpanda-data/observability/tree/main/grafana-dashboards

This issue brings these into rpk, ideally replacing rpk generate grafana-dashboard with an ability to download the latest dashboard from GitHub or serve a static one in the absence of internet connectivity.

What are the success criteria?

Improved dashboards able to be generated for Grafana via rpk.

Why is solving this problem impactful?

Improved observability of Redpanda for customers

Additional notes

It should include the 5 new dashboards and an ability to select (via flags) which dashboard.

Example of a dashboard raw file: https://raw.githubusercontent.com/redpanda-data/observability/main/grafana-dashboards/Redpanda-Ops-Dashboard.json