Created method that would allow to programmatically add new slave templates to an EC2Cloud

[drullar]

PR Description:

• Added a method which would access the templates SlaveTemplate list and add a new one if a template with the same description doesn't already exist.

• Added Unit test to verify that the new SlaveTemplate has been added successfully

Use Case:

We have a few packer jobs that create new slave template agents on demand, the issue with the current state of the ec2-plugin (prior to the changes) is that whenever we run a packer job someone will have to go and manually add the new SlaveTemplete to the EC2Cloud.
I found out that there was a partial solution in this PR #154, but it assumes that there is a SlaveTemplate to modify, where in our case we need to add new templates.

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Local Test Result:

Tests run: 165, Failures: 0, Errors: 0, Skipped: 1
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[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  09:03 min
[INFO] Finished at: 2023-04-25T13:50:39+03:00
[INFO] ------------------------------------------------------------------------

Possible Relevant Jira Issue
https://issues.jenkins.io/browse/JENKINS-71002