FIO-8912: update validateMultiple to account for model types

[brendanbond]

Link to Jira Ticket

https://formio.atlassian.net/browse/FIO-8912

Description

Some work was done to consolidate normalization of components with multiple: true. However, this doesn't account for things like hidden components or custom components, which can contain "array"-like data models that previously would get validated as multiple. This PR changes the core library's model type evaluation to (a) default to an opt-in model type in the component JSON, and (b) fallback to known component model types.

Breaking Changes / Backwards Compatibility

n/a

Dependencies

n/a

How has this PR been tested?

Added some automated testing around getModelType.

Checklist:

• I have completed the above PR template
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• My changes generate no new warnings
• My changes include tests that prove my fix is effective (or that my feature works as intended)
• New and existing unit/integration tests pass locally with my changes
• Any dependent changes have corresponding PRs that are listed above