Add Sentinel +switch-master linstener option to monitor failovers.

[ManelCoutinhoSensei]

Description of change

Fix for #3867 inspired by the redis-go library.
This solution adds a thread that cycles through valid sentinel clients to listen for +switch-master events whenever a failover happens and the user decided to monitor it.

It also provides a first step to implement a solution for #3543.

Pull Request check-list

Please make sure to review and check all of these items:

• Do tests and lints pass with this change?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

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[petyaslavova]

Hi @ManelCoutinhoSensei, thank you for your contribution! We will have a look at it.