What does this PR do?

Allow DatadogTaskListenerDecorator to be serializable. It should prevent information loss about the Charset and fix #182

Description of the Change

Jenkins sometimes serializes whole classes to send them to the jenkins agents JVM. During that process the run instance is null-ed (because not serializable and marked as transient). BuildData however is serializable, so we add the charset info to to this class, and use it instead of WorkflowRun.

Possible Drawbacks

Charset is not serializable either, so we store the canonical name in BuildData. However it might not cover all the use cases.
Other plugins like Logstash are also relying on the canonical name only.

Verification Process

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.