appsync: Unable have 2 or more API Keys (GraphqlApi construct)

### Describe the issue

By using the L2 Construct `GraphQLApi` we cannot create multiple API_KEYs, if we try to do so we get this error

```typescript
if (modes.filter((mode) => mode.authorizationType === AuthorizationType.API_KEY).length > 1) {
throw new Error('You can\'t duplicate API_KEY configuration. See https://docs.aws.amazon.com/appsync/latest/devguide/security.html');
}
```

My current workaround is this, I consider it a horrible thing to do but in the meantime is what I need.

```typescript
const expires = cdk.Expiration.after(
cdk.Duration.days(365)
).toEpoch();
const firstApiKey = new appsync.CfnApiKey(this, "FirstApiKey", {
apiId: this.apiId,
description: "First Light API Key",
expires,
});
const secondApiKey = new appsync.CfnApiKey(this, "SecondApiKey", {
apiId: this.apiId,
description: "Second API Key",
expires,
});

this.addSchemaDependency(firstApiKey);
this.addSchemaDependency(secondApiKey);

if (!this.modes.includes(appsync.AuthorizationType.API_KEY)) {
const authenticationProvider: appsync.CfnGraphQLApi.AdditionalAuthenticationProviderProperty =
{ authenticationType: appsync.AuthorizationType.API_KEY };
// eslint-disable-next-line @typescript-eslint/no-unsafe-member-access
const cfnGraphQLApi = (this as any)
.api as unknown as appsync.CfnGraphQLApi;
const additionalAuthenticationProviders: typeof cfnGraphQLApi.additionalAuthenticationProviders =
[authenticationProvider];

if (
cfnGraphQLApi.additionalAuthenticationProviders !==
undefined
) {
if (
Array.isArray(
cfnGraphQLApi.additionalAuthenticationProviders
)
) {
additionalAuthenticationProviders.push(
...cfnGraphQLApi.additionalAuthenticationProviders
);
} else {
additionalAuthenticationProviders.push(
cfnGraphQLApi.additionalAuthenticationProviders
);
}
}
cfnGraphQLApi.additionalAuthenticationProviders =
additionalAuthenticationProviders;
}
```

### Links

I have not seen anything in this [link](https://docs.aws.amazon.com/appsync/latest/devguide/security.html) saying that multiple API Keys are a problem. Furthermore, I think in that case it should be a `cdk_nag` rule instead or a warning in CDK but not an error.

Begin at the GraphQLApi construct's API_KEY validation; the report's CfnApiKey workaround and AppSync security link are the available entry points. Verify the supported multiple-key configuration and make the construct synthesize it without throwing, with regression coverage for two keys.

Written by the indexing model from the issue text.