pubsub: fix typescript types

[wemeetagain]

Unfortunately, #66, as merged, had several errors with the typescript types:

• the globalSignaturePolicy type was set as SignaturePolicy, rather than string
  • the idea is that SignaturePolicy is the "type" of the enum, which is how this would be described if this was written in typescript. Unfortunately, this type was not exported so there was a type mismatch.
• the checked-in typescript declaration files were not updated

This PR rectifies both issues

[vasco-santos]

LGTM!