Time-Shifted Comparisons in Dashboard Widgets

### Problem Statement

Currently it is not possible in Dashboards to compare a metric’s current value against a previous time period (e.g. 7 days ago, previous week, previous month) within a single widget.

While time-series charts allow users to visually inspect trends, there is no built-in way to display “value X time ago” which would allow for a direct comparison within the same widget to more easily visualize whether a metric has improved or regressed. Users must manually interpret charts

Requested in internal ticket [#181220](https://sentry.zendesk.com/agent/tickets/181220)

### Solution Brainstorm

Allow queries to be offset by a relative time window IE -7d, -1w, -1mo
This would enable comparing current vs historical values in the same widget and overlaying shifted series in time-series charts.

Another option would be to allow formulas using shifted queries for example:
current - previous
(current / previous) - 1

### Product Area

Dashboards

No files, tests, or entry points are named. Start by reviewing the Dashboards query and widget flow, then clarify whether relative offsets or shifted-query formulas are intended; done means a widget can compare current and prior periods.

Written by the indexing model from the issue text.