[Feature] Enable custom incremental strategies for dbt-spark

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

- [x] I have read the [expectations for open source contributors](https://docs.getdbt.com/docs/contributing/oss-expectations)
- [x] I have searched the existing issues, and I could not find an existing issue for this feature
- [x] I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

### Describe the feature

We currently validate that the user's incremental strategy is a built-in for `dbt-spark`. In other adapters we allow users to provide their own strategy. Additionally, this makes it more difficult for downstream Spark adapters (e.g. dbt-databricks) to implement custom strategies. They will need to override components to avoid the hard check, which then blocks them from taking future updates of those components.

### Describe alternatives you've considered

The short term workaround is to overwrite the validation macro.

### Who will this benefit?

Users of `dbt-spark` and `dbt-databricks` (maybe more?) who want to use custom incremental strategies as well as maintainers of these adapters who would like to offer this feature.

### Are you interested in contributing this feature?

_No response_

### Anything else?

_No response_