core: Allow adding custom types to check CloudFormation-Validate::F3006

### Describe the feature

After #38412 the check would only validate `AWS::` and I would like to opt-in to checking resources in a custom namespace, like `My::Resource::Example`, against a list of types I define.

### Use Case

By defining additional types I could check for typos or unknown types of a well-documented namespace.

I think it would be beneficial, if the opt-in can be done on a namespace by namespace basis. So that I am not forced to define all types I use, deal with a bunch of warnings of a namespace whose types I did not define or acknowledge F3006 and loose the feature alltogether.

### Proposed Solution

I would like some statically accessible method to define types.

I think having to create a custom `CloudFormationValidatePlugin` would not be beneficial for now, as it is hard to instantiate it once per app to replace the one that is [added by default](https://github.com/aws/aws-cdk/blob/v2.262.0/packages/aws-cdk-lib/core/lib/private/synthesis-validation.ts#L213).

### Other Information

I would like to use this as part of a cdk library. In that library I am sharing (L1, L2 and L3) Constructs with a custom namespace and I would like user's of said library to rely on the types I defined for that namespace.

Since the L1 constructs are generated by jsii, I am looking forward to any suggestions, when to call the proposed method to add defined types. Or include it in the generated code by jsii itself.

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS CDK Library version (aws-cdk-lib)

2.262.0

### AWS CDK CLI version

2.1132.0

### Environment details (OS name and version, etc.)

macOS Version 26.5.2 (25F84)

Start by reading packages/aws-cdk-lib/core/lib/private/synthesis-validation.ts, especially the CloudFormationValidatePlugin registration linked in the issue, and trace how F3006 receives its resource types. Done means a library can opt in to custom namespaces and types through a statically accessible mechanism, while unrelated namespaces do not produce warnings.

Written by the indexing model from the issue text.