fix(iam): AccountPrincipal accepts values which aren't account IDs (#20292)

Changed the type of accountId in AccountPrincipal constructor to string from any fixes #20288 


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Author: tejasmr

Diff for: packages/@aws-cdk/aws-iam/lib/principals.ts

@@ -394,6 +394,9 @@ export class AccountPrincipal extends ArnPrincipal {
    */
   constructor(public readonly accountId: any) {
     super(new StackDependentToken(stack => `arn:${stack.partition}:iam::${accountId}:root`).toString());
+    if (!cdk.Token.isUnresolved(accountId) && typeof accountId !== 'string') {
+      throw new Error('accountId should be of type string');
+    }
     this.principalAccount = accountId;
   }
 

Diff for: packages/@aws-cdk/aws-iam/test/principals.test.ts

@@ -294,6 +294,10 @@ test('AccountPrincipal can specify an organization', () => {
   });
 });
 
+test('Passing non-string as accountId parameter in AccountPrincipal constructor should throw error', () => {
+  expect(() => new iam.AccountPrincipal(1234)).toThrowError('accountId should be of type string');
+});
+
 test('ServicePrincipal in agnostic stack generates lookup table', () => {
   // GIVEN
   const stack = new Stack();