Add functions to implement Thanos Receive split functionality

[bill3tt]

This PR implements the Thanos Receive split functionality in kube-thanos.

• [x] I added CHANGELOG entry for this change.
• [ ] Change is not relevant to the end user.

Changes

Fixes #229

This PR adds a two new helper functions receiveRouter and receiveIngestor to the thanos.libsonnet library to implement the Thanos Receive split functionality.

NB: This was the first time I have properly used Jsonnet - and I had a horrible time. Therefore I've almost certainly broken unwritten rules and are generally holding it wrong. Please help correct anything that is egregiously wrong

Verification

• Applied the generated manifests from example.jsonnet to a local kind cluster.
• Used the up binary to remote write some data.
• Verified that the data showed up in the Thanos Query UI.

[bill3tt]

Thanks for your speedy review @kakkoyun :raised_hands: I think I've addressed all the comments :)