Selectable / configurable timewindow for Linkerd dashboard

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## Feature Request

### What problem are you trying to solve?
The linkerd dashboard shows near real-time data for most aggregated views (i.e. RPS, errors etc.). Some of this is also accessible as time series views via Granfana views, but other views such as dependency graphs etc. can not be configured to display the behaviour over a particular time window.

On some environments that don't see consistent load (i.e. pre production) this means that certain views don't always show interesting/insightful info because the default time window is too short or not configurable.

Ideally it would be great if there was a way via the dashboard to either select a "last X" window that all metrics/views are aggregated under (i.e. last 10 mins, last 1 hour, etc.), or even better a way to set a defined time window (i.e. between 1pm - 2pm) also.

This would be particuarily useful for cases such as:

- Examining behaviour during performance/load tests, where you only want to consider a defined time window.
- Instances where you have changed dependencies between services and you want to be able to compare dependency graphs (and other metrics) before and after the change was rolled out.
- In situations where you know activity is likely to deviate from the norm at a defined time (i.e. new content/feature release) and you want to observe behaviour at/from that point.

### How should the problem be solved?

Ideally; a menu added to the dashboard, that lets you select a time window for data aggregation (similar to what Granfana exposes). Supporting multiple types of time window i.e. last X, from X, between X and Y, ... and options for refresh frequency (only applies to "last X" and "from X").

### Any alternatives you've considered?

N/A

### How would users interact with this feature?

- User opens dashboard
- Default view is (near) real-time, for example last 1 minute with an auto refresh frequency of 10 seconds (or whatever linkerd dashboard currently does).
- User can select from a set of menus (similar to Grafana) an alternative time window to view aggregated metrics between, and an alternative update freqeuency (for relevant time windows).

The issue names no files, tests, or entry points; begin by locating the Linkerd dashboard implementation and its current aggregation and refresh behavior. Done means users can choose relative or fixed time windows and applicable refresh frequencies while preserving the default near-real-time view.

Written by the indexing model from the issue text.