Add prop to override color selection in gauge components

Signed-off-by: James Turley <jamesturley@gocardless.com>

.changeset/grumpy-pugs-report.md

@@ -0,0 +1,23 @@
+---
+'@backstage/core-components': patch
+---
+
+Add new way to override color selection to progress bar/gauge components.
+
+`Gauge`, `LinearGauge` and `GaugeCard` all accept a `getColor` prop,
+which is a function of the type:
+
+```ts
+export type GetColor = (args: {
+  palette: Palette;
+  value: number;
+  inverse?: boolean;
+  max?: number;
+}) => string | PaletteColor;
+```
+
+Either return a standard Material UI palette color object or a CSS color
+string (e.g. "red", "#f02020"), and the gauge will be set to that color.
+
+If the prop is omitted, the default implementation is unchanged from previous
+versions.