### What changes were proposed in this pull request?
When SSL is enabled for master, secure the Ratis communication as well with TLS
### Why are the changes needed?
Currently, when TLS is enabled for RPC, Ratis comms still goes in the clear - add support for TLS.
Note that currently this only supports GRPC, and not netty.
### Does this PR introduce _any_ user-facing change?
Secures ratis communication when TLS is enabled at master for rpc.
### How was this patch tested?
Local tests and additional unit tests added
Closes#2515 from mridulm/CELEBORN-1401-add-ratis-ssl-support.
Authored-by: Mridul Muralidharan <mridulatgmail.com>
Signed-off-by: Shuang <lvshuang.xjs@alibaba-inc.com>
### What changes were proposed in this pull request?
User documentation for configuring SSL and authentication
### Why are the changes needed?
Document the new features.
### Does this PR introduce _any_ user-facing change?
Yes, updates documentation
### How was this patch tested?
Used `mkdocs serve` to render and validate the documentation.
Closes#2481 from mridulm/ssl-auth-documentation.
Lead-authored-by: Mridul Muralidharan <mridul@gmail.com>
Co-authored-by: Mridul Muralidharan <mridulatgmail.com>
Signed-off-by: zky.zhoukeyong <zky.zhoukeyong@alibaba-inc.com>
