Java & Kotlin - type mapping support in the asyncapi cli

### Why do we need this improvement?

As Kotlin user, would appreciate to have the data mapping support so that I could map type such as Double -> BigDecimal to avoid possible side-effects.
Double -> BigDecimal is the only issue with type mapping we had, but considering other users might have different use cases, I am creating the ticket for general type mapping.

### How will this change help?

It will help Java & Kotlin users to have type mapping so they don't need to use default mapping of types.

### Screenshots

_No response_

### How could it be implemented/designed?

Having another flag in the `asyncapi generate model` to support type mapping, something like `--kotlinTypeMapping`

### 🚧 Breaking changes

No

### 👀 Have you checked for similar open issues?

- [x] I checked and didn't find a similar issue

### 🏢 Have you read the Contributing Guidelines?

- [x] I have read the [Contributing Guidelines](https://github.com/asyncapi/.github/blob/master/CONTRIBUTING.md)

### Are you willing to work on this issue?

None

Start at the `asyncapi generate model` CLI entry point and inspect how Java and Kotlin generators currently select default types. Define the mapping option and verify Double → BigDecimal plus other custom mappings through generator tests; done means both targets accept mappings without changing default behavior.

Written by the indexing model from the issue text.