Support stateless Ruler

[saswatamcode]

This PR adds support for running Stateless Rulers using kube-thanos.

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

Changes

• Adds option remoteWriteConfigFile to defaults in kube-thanos-rule.libsonnet
• Modifies all.jsonnet to use newly added option

Verification

Tested locally.