Remove `thanosSelector` and expose `mixin._config.thanos` config variable for thanos sidecar

This commit removes existing `thanosSelector` and exposes a single config variable `mixin._config.thanos` to customize thanos sidecar mixins. It follows same structure as https://github.com/thanos-io/thanos/blob/d2d74dac9878f633f1fb0e253bcc0b1cdea376d3/mixin/config.libsonnet



Signed-off-by: Arunprasad Rajkumar <arajkuma@redhat.com>