Add Credentials Support with credentials select widget [JENKINS-35503]

[kmadel]

Most importantly, this is a fix for JENKINS-35503. Based on work done in PR #208
Example of integrating with xmlns:c="/lib/credentials" c:select for selecting credentials. Feel free to merge or not.

Create credential set:

Apply to Slack configuration:

[kmadel]

Looking for two thumbs up - and then we will get this merged and released.

[samrocketman]

cc @fgs-dbudwin
edit: currently upgrading my scripts to test this change.

[samrocketman]

👍 from me

My testing is completed:

• PASS -Sucessfully build this pull request and pass unit tests
• PASS - Upgrade from Slack plugin 2.0.1 to this pull request version and see how upgrades behave.
• PASS - Configure new credentials ID with the slack plugin and test with it. It should behave the same.
• PASS - Test connection button with credentials ID configured.

How was testing prepared?

git clone https://github.com/samrocketman/jenkins-bootstrap-slack
cd jenkins-bootstrap-slack/
#ensure that no services are listening on iface:port localhost:8080
./slack_bootstrap.sh
#follow instructions to log into Jenkins at the end of the script output

I then visited https://jenkins-slack-plugin.slack.com/ to see how slack messages were posted during my tests.