ExposureType - custom extension of EnumType

### 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

In the wild, I'm seeing lots of spreadsheets with dependencies on the data warehouse.
We like to register them as exposures for visibility.
How would we extend ExposureType to include "spreadsheet"? Is it possible without modifying dbt core?

### Describe alternatives you've considered

We use "application" but it's not intuitive. Feels pretentious.

### Who will this benefit?

report builders documenting downstream spreadsheets as dbt exposures.

### Are you interested in contributing this feature?

yes

### Anything else?

_No response_