Cross-account data sources

Hi,

I was looking to check if AppSync supports cross-account data sources, such as AppSync in one account using a DynamoDB table in another account.

As per [@dabit3's Tweet reply from April 1, 2020](https://twitter.com/dabit3/status/1245439900356411393), this is not yet supported but a workaround is available: using a custom Lambda GraphQL resolver.

Using Lambda resolvers for cross-account access is also showcased in the [Develop serverless GraphQL architectures using AWS AppSync (MOB309-R1) from re:Invent 2019](https://www.youtube.com/watch?v=XVU4pYeNfNo), with the slides [posted here](https://d1.awsstatic.com/events/reinvent/2019/REPEAT_1_Develop_serverless_GraphQL_architectures_using_AWS_AppSync_MOB309-R1.pdf).

Creating this issue for 2 reasons:
- confirm cross-account support is not a thing. Took me a while to find that Tweet
- track community desire for this feature

Start with the issue body and verify the current AWS AppSync cross-account data-source support using the linked tweet and re:Invent slides. Confirm the supported status, assess the documented Lambda resolver workaround, and record the result and community demand in the issue; done means the support question is answered clearly.

Written by the indexing model from the issue text.