[Feature request] Support for memcached as the index cache

[muffix]

Is this a request for help?:
No

Is this a BUG REPORT or FEATURE REQUEST? (choose one):
Feature request

I'd like to use Memcached instead of local memory for the store gateway. This is supported since Thanos 0.10.0.

Version of Helm and Kubernetes:
Helm: v2.16.1
Kubernetes: v1.12.8

Which chart in which version:
prometheus-thanos v2.10.0

What happened:
The chart only supports the in-memory store

What you expected to happen:
Being able to configure the type of the index cache

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know:
I'm going to raise a PR for this soon.