Add Redis configuration options for Sentinel support

[shkarface]

What does this PR do?

This PR adds support for Redis Sentinel, this ensures high availability for non-cluster redis setups.

Motivation

A recent redis outage caused our rate limiters to stop working and traefik responded with 500, this ensures that traefik can use a highly available redis setup

More

• [ ] Added/updated tests
• [x] Added/updated documentation

[nmengin]

Hello @shkarface,

Thank you for your contribution.

We've set the status to "review" to allow us to check the PR. We'll keep you updated once the analysis is done.

[shkarface]

Thank you @nmengin, looking forward to getting this merged