chore(maven) factorize metadata into a mavenmetadata package + apply code conventions for plugin

[dduportal]

This PR is a refactor of the maven plugin to allow continuing the work in #412.
No functionnal changes (hence the chore) label.

It introduces the following changes:

• Update the maven example + add it to the end 2 end testing framework
• Moved the mavenmetadata logic into its own package + use it on both maven and jenkins to decouple
  • Include unit test with HTTP mocking (reusing the work from chore: refactor resource dockerfile #453)
• Apply code conventions for the plugin maven (exports a Spec object and a New(spec) constructor method

Test

To test this pull request, you can run the following commands:

go build -o ./dist/updatecli && ./dist/updatecli diff --config examples/updatecli.generic/maven.yaml
make test-e2e

Additional Information

Jenkins plugin is the next to be refactored. Almost there \o/

[dduportal]

PR ready with your last feedbacks (and blocking #482).

[dduportal]

Thanks for the review @lemeurherve !

PR rebased with your proposal

[dduportal]

Since this PR is labelled as chore (e.g. no functionnal change), and that it has been reviewed with only minor elements, I'll put it as auto-merge with squash.

Thanks folks!