Send ssh environment variables

[laurentmorin29]

In my use case, I push some code on batch servers using gradle ssh plugin.
Recently, my SSI added the requirement of passing specific environement variables to log the reason of access.

New feature

SSH allows client to send environment variables to the target host using setEnv method on ChanelExec.
https://epaul.github.io/jsch-documentation/javadoc/com/jcraft/jsch/ChannelExec.html#setEnv-java.lang.String-java.lang.String-

Configuration

1. add a config map(string,string) of values into the gradle config to provides all variables and maybe could be merged with definitions provided on run or session

ssh.settings {
    env {
        var_1 = 'var_1 content'
        var_2 = 'var_2 content'
   }
}

Update

As I could see, the Command.groovy script could simply merge parent env configs and use setEnv for all of them on the channel

https://github.com/int128/groovy-ssh/blob/master/core/src/main/groovy/org/hidetake/groovy/ssh/operation/Command.groovy

Sorry I'm not fluent with groovy dev so I created this issue and not a PullRequest.