Proto commits in JackAppDev/flutter_iap

These 5 commits are when the Protocol Buffers files have changed:

Commit:6d8aeb9
Author:Sebastian Roth
Committer:Sebastian Roth

improve error handling (#27) and catch BillingManager initialization errors earlier

The documentation is generated from this commit.

Commit:9dc7ae3
Author:Sebastian Roth

mark most product fields optional

Commit:56dfc19
Author:Sebastian Roth

add a new "PurchaseRequest" message which will pave the way for different IAP types (single, subscriptions).

Commit:ee80912
Author:Sebastian Roth

Adds various error codes as they could appear with the google play billing API.

Commit:9bf3e79
Author:Sebastian Roth
Committer:Sebastian Roth

Initial Protobuf port of all library functions