feat(coderd/healthcheck): add health check for proxy (#10846)

Adds a health check for workspace proxies:
- Healthy iff all proxies are healthy and the same version,
- Warning if some proxies are unhealthy,
- Error if all proxies are unhealthy, or do not all have the same version.

coderd/apidoc/docs.go

@@ -12388,6 +12388,9 @@ const docTemplate = `{
                 },
                 "websocket": {
                     "$ref": "#/definitions/healthcheck.WebsocketReport"
+                },
+                "workspace_proxy": {
+                    "$ref": "#/definitions/healthcheck.WorkspaceProxyReport"
                 }
             }
         },
@@ -12427,6 +12430,29 @@ const docTemplate = `{
                 }
             }
         },
+        "healthcheck.WorkspaceProxyReport": {
+            "type": "object",
+            "properties": {
+                "error": {
+                    "type": "string"
+                },
+                "healthy": {
+                    "type": "boolean"
+                },
+                "severity": {
+                    "$ref": "#/definitions/health.Severity"
+                },
+                "warnings": {
+                    "type": "array",
+                    "items": {
+                        "type": "string"
+                    }
+                },
+                "workspace_proxies": {
+                    "$ref": "#/definitions/codersdk.RegionsResponse-codersdk_WorkspaceProxy"
+                }
+            }
+        },
         "netcheck.Report": {
             "type": "object",
             "properties": {