Functions: support dynamic express route parameters

[njbmartin]

Looking at this PR, named splat routes were added:
#33301

eg. /api/customers/[...customerId].ts

This is great, but it doesn't look like dynamic routes in Functions are fully supported yet.

Moving from a pure express API to Gatsby Functions, I can't seem to do /api/auth/[customerId]/[customerEmail].ts as these return undefined.

[njbmartin]

Actually, this might be because it's not yet supported in Gatsby Cloud?

[KyleAMathews]

Hmm does it work locally?

[njbmartin]

Yes, it works locally, just not in Cloud so had to revert the build