resolves #620 by adding maven sync command

[kowalczykp]

• I signed JFrog's CLA.
• All tests passed. If this feature is not already covered by the tests, I added new tests.
• This pull request is on the dev branch.
• I used gofmt for formatting the code before submitting the pull request.

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This adds maven sync command to the cli, which was added in the jfrog/jfrog-client-go#154

invocation: jfrog-cli bt ms subject/repository/package/version

help:

Name:
  jfrog bt maven-central-sync - Sync Version Artifacts to Maven Central

Usage:
  jfrog bt mcs [command options] <target path>

Arguments:
  target path
    The path, in Bintray, to the version that should be synced.
    Format: subject/repository/package/version.

Options:
  --user                 [Optional] Bintray username. If not set, the subject sent as part of the command argument is used for authentication.
  --key                  [Mandatory] Bintray API key
  --sonatype-username    [Optional] Sonatype OSS username
  --sonatype-password    [Optional] Sonatype OSS user password
  --dont-close           [Default: false] By default the staging repository is closed and artifacts are released to Maven Central.

[kowalczykp]

@eyalbe4 @Or-Gabay did you get any chance to look at this PR? Im happy to make any required changes to get this accepted

[bootstraponline]

@eyalb4doc @liron-shalom @barbelity Hey, it'd be great to have the maven sync feature reviewed and merged.

[bootstraponline]

@eyalb4doc @liron-shalom @barbelity If there's anything I can do to help get this reviewed and merged, let me know.

[kowalczykp]

@eyalbe4 I've addressed the PR review comments and squashed the changes

[CLAassistant]

All committers have signed the CLA.