RFR: 7384: Add option to enable STARTTLS encryption for mail server in communication preference

[Suchita Chaturvedi]

This PR addresses a new enhancement of adding the preference for STARTTLS encryption for the email communications triggered in JMX Console. 

<img width="314" alt="starttls" src="https://user-images.githubusercontent.com/11155712/128935485-139b4558-c6aa-406c-92df-cc78883c8f0c.png">

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Commit messages:
 - 7384: Add option to enable STARTTLS encryption for mail server in communication preference

Changes: https://git.openjdk.java.net/jmc/pull/303/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=303&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7384
  Stats: 24 lines in 6 files changed: 18 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jmc/pull/303.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/303/head:pull/303

PR: https://git.openjdk.java.net/jmc/pull/303