[Feat]: Jenkins CI attestor

[joshdabosh]

Describe the solution you'd like:

Capture Jenkins CI environment variables

User value: We'll be able to query attestations using Jenkins build data. This will also allow Jenkins data to be used in the SLSA provenance attestor.

Expected behavior: Jenkins attestor will capture default Jenkins environment variables.

Proposed solution: Pretty much the same as the Github / GitLab attestors.

Anything else you would like to add:

Testing changes required:

Documentation changes required: Add the Jenkins attestor to documentation.