feat(core): allow overriding the stage name (aws#22223)

Currently the stage name is derived from the id of the stage and later becomes part of each stack name inside the stage:

https://github.com/aws/aws-cdk/blob/c425e8ca1a3d296eb6a7fd7e005d07c1eadd16aa/packages/%40aws-cdk/core/lib/stage.ts#L139

https://github.com/aws/aws-cdk/blob/c425e8ca1a3d296eb6a7fd7e005d07c1eadd16aa/packages/%40aws-cdk/core/lib/stack.ts#L1139-L1143

Since the ids in a scope must be different, deploying the same stage in the same scope to multiple regions would require different ids and would then lead to different stack names in the end. To keep stack names uniform across regions, a stack already allows customising the stack name uncoupled from the id:

https://github.com/aws/aws-cdk/blob/c425e8ca1a3d296eb6a7fd7e005d07c1eadd16aa/packages/%40aws-cdk/core/lib/stack.ts#L106-L111

This PR follows up on that concept and also allows customising the stage name.

This would then allow to deploy the same stage in the same scope to multiple regions while keeping the resulting stack names the same across regions.

----

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/core/lib/stage.ts

@@ -53,6 +53,13 @@ export interface StageProps {
    * temporary directory will be created.
    */
   readonly outdir?: string;
+
+  /**
+   * Name of this stage.
+   *
+   * @default - Derived from the id.
+   */
+  readonly stageName?: string;
 }
 
 /**
@@ -136,7 +143,7 @@ export class Stage extends Construct {
     this.account = props.env?.account ?? this.parentStage?.account;
 
     this._assemblyBuilder = this.createBuilder(props.outdir);
-    this.stageName = [this.parentStage?.stageName, id].filter(x => x).join('-');
+    this.stageName = [this.parentStage?.stageName, props.stageName ?? id].filter(x => x).join('-');
   }
 
   /**

packages/@aws-cdk/core/test/stage.test.ts

@@ -92,6 +92,17 @@ describe('stage', () => {
     expect(stack.stackName).toEqual('MyStage-MyStack');
   });
 
+  test('can override the stage name', () => {
+    // WHEN
+    const app = new App();
+    const stage = new Stage(app, 'NotMyStageName', { stageName: 'MyStage' });
+    const stack = new BogusStack(stage, 'MyStack');
+
+    // THEN
+    expect(stage.stageName).toEqual('MyStage');
+    expect(stack.stackName).toEqual('MyStage-MyStack');
+  });
+
   test('Can not have dependencies to stacks outside the nested asm', () => {
     // GIVEN
     const app = new App();