Option to allow TLS based communication with Zookeeper Backend

[palsaurabh2005]

Is your feature request related to a problem? Please describe.
Zookeeper supports TLS based communication with clients. Vault should also be able to provide an option using configuration to enable TLS based communication with a Zookeeper Backend.

Describe the solution you'd like
Based on TLS related properties defined in the Zookeeper backend 'stanza' in Vault config, Vault should enable a TLS context to talk to Zookeeper backend.

Explain any additional use-cases
When verifying the Certificate presented by the Zookeeper backend, Vault should be able to verify the certificate's CN/SAN using either the IP address or DNS. Option to do so will be provided using a config flag. Default would be the DNS name.

Additional context
There is a closed issue pertaining to the same feature request(#1652). I am submitting this as a new issue, since I have not been able to gather attention to the pull request for this feature request.

CC: @devth @sherzberg @kenbreeman @elupu @reegz

[palsaurabh2005]

PR for the feature request: #4856
Documentation for Config options: https://github.com/palsaurabh2005/vault/blob/feature/vault-zk-mtls-support/website/source/docs/configuration/storage/zookeeper.html.md

[jefferai]

Closing, essentially a duplicate of the PR the author previously made.