Add flat-square action badge style (#34062)

Adds the `flat-square` style to action badges. Styles can be selected by
adding `?style=<style>` to the badge endpoint. If no style query is
given, or if the query is invalid, the style defaults to `flat`.

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/shared/actions/runner_badge_flat-square.tmpl

@@ -0,0 +1,15 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="{{.Badge.Width}}" height="20"
+	role="img" aria-label="{{.Badge.Label.Text}}: {{.Badge.Message.Text}}">
+	<title>{{.Badge.Label.Text}}: {{.Badge.Message.Text}}</title>
+	<g shape-rendering="crispEdges">
+		<rect width="{{.Badge.Label.Width}}" height="20" fill="#555" />
+		<rect x="{{.Badge.Label.Width}}" width="{{.Badge.Message.Width}}" height="20" fill="{{.Badge.Color}}" />
+	</g>
+	<g fill="#fff" text-anchor="middle" font-family="{{.Badge.FontFamily}}"
+		text-rendering="geometricPrecision" font-size="{{.Badge.FontSize}}">
+		<text x="{{.Badge.Label.X}}" y="140"
+			transform="scale(.1)" fill="#fff" textLength="{{.Badge.Label.TextLength}}">{{.Badge.Label.Text}}</text>
+		<text x="{{.Badge.Message.X}}" y="140" transform="scale(.1)" fill="#fff"
+			textLength="{{.Badge.Message.TextLength}}">{{.Badge.Message.Text}}</text>
+	</g>
+</svg>