Add logic to convert from Legacy Events to CloudEvents.

[eamonnmcmanus]

This allows received Legacy Events to be handled by function code that expects a
CloudEvent input. The translation logic and tests are based closely on the
corresponding code in https://github.com/GoogleCloudPlatform/functions-framework-dotnet.

The new code also passes the relevant conformance tests in
https://github.com/GoogleCloudPlatform/functions-framework-conformance, once
GoogleCloudPlatform/functions-framework-conformance#30
is applied. For now I'm running those tests manually. I plan at least to commit
the script and test functions that allow me to do so.

[eamonnmcmanus]

My main concern here is about the maybeReshapeData method, which I added based on the dotnet original. I had to deduce what the resultant CloudEvent looks like, since I didn't see anything in the dotnet repo that tested it. But I might not have been looking hard enough.

[jskeet]

My main concern here is about the maybeReshapeData method, which I added based on the dotnet original. I had to deduce what the resultant CloudEvent looks like, since I didn't see anything in the dotnet repo that tested it. But I might not have been looking hard enough.

Most of the testing is in https://github.com/GoogleCloudPlatform/functions-framework-dotnet/blob/master/src/Google.Cloud.Functions.Framework.Tests/GcfEvents/EventDeserializationTest.cs (and the test files in that directory)

But for the actual shape of the CloudEvent, is your friend.

[jskeet]

Looks okay to me, although there may well be subtleties that I've missed. I've added a comment for a simplification you can probably make.

There will be more Firestore/Firebase work to do in terms of source and subject - I'm hoping to get the proposed CloudEvent format finalized soon. (Will include you in the doc.)

[eamonnmcmanus]

Most of the testing is in https://github.com/GoogleCloudPlatform/functions-framework-dotnet/blob/master/src/Google.Cloud.Functions.Framework.Tests/GcfEvents/EventDeserializationTest.cs (and the test files in that directory)

Oh, excellent! It looks as if I can get better test coverage just by translating that test to Java, which I'll do in a later change.

But for the actual shape of the CloudEvent, is your friend.

Is there a word missing here?

[jskeet]

Whoops - I could have sworn I copied the link. Doh. https://github.com/googleapis/google-cloudevents is the source of truth for the format of the data part within Google CloudEvents. (You may wish to consume that via https://github.com/googleapis/google-cloudevents-java when the code is published appropriately, in order to test the conversion.)