Document that Workflow::Validator is an interface contract

[ehuelsmann]

Description

Declaring the Workflow::Validator an interface contract as opposed to a
base class for validators allows any class to implement validations. This
addresses a long-standing desire of the OpenXPKI project to consolidate
the Condition and Validator concepts - in order to be able to share code
between the two - without actually consolidating them.

Closes #137

Type of change

• New feature (non-breaking change which adds functionality)
• Breaking change

This is really meant to be a documentation-only change. However, it didn't actually pan out that way, because of removing both the documentation and implementation of the _init() method: there's no place for a "dummy" method in an interface definition. Also, it doesn't make sense from an OOP perspective to have that method. Instead, a newly declared init() method should be calling its parent's init() as with regular OOP patterns.

Checklist:

• My code follows the style guidelines of this project, please see the contribution guidelines.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

This is a good point: should we add tests which implement a validator as a blessed ref not derived from Workflow::Validator or Workflow::Base?

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[coveralls]

Coverage decreased (-0.02%) to 92.431% when pulling e9471a4 on validator-as-interface into 0138f62 on master.

[jonasbn]

Looks good.

I believe we need to add Moo to the cpanfile

[jonasbn]

LGTM

Please ignore my previous comment, I missed it was in the documentation part

[ehuelsmann]

@jonasbn As far as I'm concerned, we're ready to merge this.

[oliwel]

I did not check the related code details from the general idea behind I really vote for this!

[ehuelsmann]

Merged based on all prior approvals.