[source][condition] Add Jenkins resource [source,condition]

[olblak]

Add a new Jenkins resource type to manipulate Jenkins version information.
This PR is inspired from garethjevans/jenkins-versions

kind: jenkins
spec:
    release: <specify release name, stable or weekly>
    version: <specify a version>, default value set to source output
    github:
        username: <needed to retrieve changelog from github>
        token: <needed to retrieve changelog from github>

source
The jenkins source retrieves the jenkins version based on its release type, either [weekly.latest]
This resource fetch the changelog information from https://github.com/jenkinsci/jenkins

condition
The Jenkins condition test a

To test

export UPDATECLI_GITHUB_TOKEN=xxx
make build && ./bin/updatecli diff --config examples/updateCli.generic/jenkins --values examples/values.yaml

Signed-off-by: Olivier Vernin olivier@vernin.me