api-gateway: access custom cognito claim in parameterMapping of HttpAlbIntegration

### Describe the bug

When declaring a HttpAlbIntegration with parameterMapping for a regular cognito claim, say email or sub, everything works fine. But when defining a parameterMapping for a custom attribute, where the sintaxt implies using the prefix `custom:`, it complains about being an invalid expression.

I've tried according the RestApi documentation (https://docs.aws.amazon.com/apigateway/latest/developerguide/apigateway-enable-cognito-user-pool.html) but no luck:
`"$context.authorizer.claims['custom:my_attr']"`

### Expected Behavior

The custom attribute should be accessible like any other.

### Current Behavior

cdk returns a BadRequest:
```
Invalid mapping expression specified: Validation Result: warnings : [], errors :
[Invalid mapping expression specified: $context.authorizer.claims.custom:my_attr]
(Service: AmazonApiGatewayV2; Status Code: 400; Error Code: BadRequestException; Request ID: d9f58770-dc40-4268-9515-ef4cc354e4d3; Proxy: null)
```

### Reproduction Steps

Create a HttpAlbIntegration for the api gateway:
```
const integration = new HttpAlbIntegration('http-alb-integration', albListener, {
method: HttpMethod.ANY,
vpcLink: vpcLinkStack.vpcLink,
parameterMapping: new ParameterMapping()
.appendHeader('x-my-attr', MappingValue.contextVariable('authorizer.claims.custom:my_attr'))
});
```

cdk will complain about the `authorizer.claims.custom:my_attr` because of the `:`.

### Possible Solution

Some alternative sintax for these cases, for instance: `authorizer.claims.[custom:my_attr]`
so `custom:my_attr` would be treated as a block.

### Additional Information/Context

_No response_

### CDK CLI Version

2.41.0 (build 6ad48a3)

### Framework Version

_No response_

### Node.js Version

v16.14.2

### OS

macOS Monterey 12.5.1

### Language

Typescript

### Language Version

3.9.7

### Other information

_No response_

Start with the HttpAlbIntegration, ParameterMapping, and MappingValue.contextVariable entry points shown in the reproduction, then reproduce the Amazon API Gateway HTTP API BadRequest with the custom:my_attr claim. Trace where authorizer claim expressions are validated or assembled. Done means a custom Cognito claim can be mapped to x-my-attr without an invalid-expression error, with coverage for the reported mapping.

Written by the indexing model from the issue text.