Add New API in Flow Model to Check Sub-Types

### Description

A new API needs to be introduced in `TypeManagerService` to check for sub-types, as discussed in https://github.com/wso2/product-ballerina-integrator/issues/752#issuecomment-3280413391.

Draft design:
```java
public boolean isSubType(String fileName, Type sourceType, Type targetType)
```
> Note: `Type` - definition yet to be finalized

### Version

_No response_