raw_request.Client + V2 UBB Examples

[jar-stripe]

Why?

Stripe SDK for go does not have directly support V2 APIs but it is still possible to access them for e.g. Usage Based Billing. This PR contains examples that show how to use the rawrequest functionality to handle the new events and endpoints. Because the high-throughput meter event stream requires changing both the backend and the API key, this PR also introduces a raw_request.Client struct that behaves like other services.

What

• added raw_request.Client which accepts a RawRequestBackend and API key; this can be different than stripe.Key or stripe.APIBackend
• added support for a MeterEvents backend to stripe, to enable posting meter events
• added thinevent_webhook_handler.go which receives a thin event, gets the v2 Event and then the related object for a v1.billing.meter.error_report_triggered event
• added meter_event_stream.go which demonstrates how to utilize the meter event stream for high throughput meter events

[ramya-stripe]

@jar-stripe Can you look at the test failures please?

[jar-stripe]

@jar-stripe Can you look at the test failures please?

@ramya-stripe Tests pass, and I updated the title and PR description to better represent the scope of the PR. If you agree with my reasoning above re: using rawrequest.Client in both examples, this should be good to go!

[ramya-stripe]

Note: We will need to do a release of the Go SDK for the example to be usable