Add support for batch api

[MrTin]

Our use case is to update user properties daily. We batch-load hundreds of users at a time from DB but send one request per user using this library.

It's not a significant pain as we are in the low thousands, but we are growing steadily, which will become a problem. I'm hoping you have a chance to add this before it does, or I'll have to resort to either submitting a PR or making a httparty :-)

Relevant news article: https://www.customer.io/docs/release-notes/2023-01-10-track-v2-api/

[ishields]

Hello, I'm in the same boat and would love to be able to send batch instead of single requests for performance reasons. This would help us avoid ever hitting the rate limits.