feat(medusa, utils): Allow object feature flags (#4701)

Feature flags can be set as follows:

**Environment variables**
```
MEDUSA_FF_ANALYTICS=true
MEDUSA_FF_WORKFLOWS=createProducts,addShippingMethods
```

**Project config**
```
{
  featureFlags: {
    analytics: true,
    workflows: {
      createProducts: true,
      addShippingMethods: true,
    }
  }
}
```

packages/medusa/src/utils/flag-router.ts

@@ -1,21 +1,72 @@
-import { FeatureFlagsResponse, IFlagRouter } from "../types/feature-flags"
+import { FeatureFlagTypes } from "@medusajs/types"
+import { isObject, isString } from "@medusajs/utils"
 
-export class FlagRouter implements IFlagRouter {
-  private readonly flags: Record<string, boolean> = {}
+export class FlagRouter implements FeatureFlagTypes.IFlagRouter {
+  private readonly flags: Record<string, boolean | Record<string, boolean>> = {}
 
-  constructor(flags: Record<string, boolean>) {
+  constructor(flags: Record<string, boolean | Record<string, boolean>>) {
     this.flags = flags
   }
 
-  public isFeatureEnabled(key: string): boolean {
-    return !!this.flags[key]
+  /**
+   * Check if a feature flag is enabled.
+   * There are two ways of using this method:
+   * 1. `isFeatureEnabled("myFeatureFlag")`
+   * 2. `isFeatureEnabled({ myNestedFeatureFlag: "someNestedFlag" })`
+   * We use 1. for top-level feature flags and 2. for nested feature flags. Almost all flags are top-level.
+   * An example of a nested flag is workflows. To use it, you would do:
+   * `isFeatureEnabled({ workflows: Workflows.CreateCart })`
+   * @param flag - The flag to check
+   * @return {boolean} - Whether the flag is enabled or not
+   */
+  public isFeatureEnabled(flag: string | Record<string, string>): boolean {
+    if (isString(flag)) {
+      return !!this.flags[flag]
+    }
+
+    if (isObject(flag)) {
+      const [nestedFlag, value] = Object.entries(flag)[0]
+
+      if (typeof this.flags[nestedFlag] === "boolean") {
+        return this.flags[nestedFlag] as boolean
+      }
+
+      return !!this.flags[nestedFlag]?.[value]
+    }
+
+    throw Error("Flag must be a string or an object")
   }
 
-  public setFlag(key: string, value = true): void {
+  /**
+   * Sets a feature flag.
+   * Flags take two shapes:
+   * setFlag("myFeatureFlag", true)
+   * setFlag("myFeatureFlag", { nestedFlag: true })
+   * These shapes are used for top-level and nested flags respectively, as explained in isFeatureEnabled.
+   * @param key - The key of the flag to set.
+   * @param value - The value of the flag to set.
+   * @return {void} - void
+   */
+  public setFlag(
+    key: string,
+    value: boolean | { [key: string]: boolean }
+  ): void {
+    if (isObject(value)) {
+      const existing = this.flags[key]
+
+      if (!existing) {
+        this.flags[key] = value
+        return
+      }
+
+      this.flags[key] = { ...(this.flags[key] as object), ...value }
+      return
+    }
+
     this.flags[key] = value
   }
 
-  public listFlags(): FeatureFlagsResponse {
+  public listFlags(): FeatureFlagTypes.FeatureFlagsResponse {
     return Object.entries(this.flags || {}).map(([key, value]) => ({
       key,
       value,