[Feature] Custom strategy for incremental models when table type is iceberg

### Is this your first time submitting a feature request?

- [X] I have searched the existing issues, and I could not find an existing issue for this feature

### Describe the feature

I would like to use a custom merge strategy for incremental models in case of `iceber` tables.
For example
```sql
{{
config(
unique_key=['id'],
materialized='incremental',
table_type='iceberg',
incremental_strategy='custom_merge',
...
)
}}
```

however, I found a restriction [here](https://github.com/dbt-athena/dbt-athena/blob/f1af36238c7e5b97c380beaba824fdfa2dee98e1/dbt/include/athena/macros/materializations/models/incremental/helpers.sql#L4) to do that, and I got

```sh
Incremental models on Iceberg tables only work with 'append' or 'merge' (v3 only) strategy.
```

Is there a main reason for this restriction?

### Describe alternatives you've considered

_No response_

### Who will this benefit?

Anyone that want to use a [custom strategy](https://docs.getdbt.com/docs/build/incremental-strategy#custom-strategies) for incremental models in `iceberg` tables.

### Are you interested in contributing this feature?

yes

### Anything else?

[custom strategy](https://docs.getdbt.com/docs/build/incremental-strategy#custom-strategies)