Add PDF export in API

[woodthom2]

Description

@narendrakumar9867 has added PDF export functionality
harmonydata/harmony#114

This fixed the issue harmonydata/harmony#53

We now need to expose the PDF export as an API endpoint.

Rationale

This is needed so that Harmony web front end and Harmony R can access the PDF export functionality

[narendrakumar9867]

@woodthom2 Noted, I have started working on the remaining API part. Thanks for the update.

[woodthom2]

Thanks so much Narendra!