Fix addEventCallback input parameter type

[tnorling]

Fixes input parameter type for addEventCallback which broke when moving from v2 to v3

Addresses #6309

[sameerag]

Can we add tests to match the type?

[peterzenz]

Anything we can do to properly test for this?

[jo-arroyo]

Thanks for this fix, Thomas!

[tnorling]

Generally speaking testing types is done by the TypeScript compiler, you can't really write unit tests against non-primitive types because types are stripped at runtime. Functionally nothing changes here, this is purely a TypeScript change.

We do have tests for the addEventCallback API, however, they continued to pass because PCA was expecting a less specific type. Now that we've restored the expectation for a more specific type attempts to pass in a less specific type would fail at the TypeScript level.