[#112] Enable GPG checks of channel artifacts

[spyrkob]

Fixes #112

• Initial SignatureValidator API
• Pass GPG setting to dependant channels
• Update validator signatur
• Add SignatureValidator implementation
• Use return status instead of exceptions
• Report validation errors
• Add keyserver support to the validator
• Create channel version 2.1.0

[spyrkob]

@jmesnil could you review this please? It's a large one, if it makes it easy I can split it up into spec/API changes and implementation PRs.

[jmesnil]

do we need this method as it kind of duplicates isGpgCheck?

[spyrkob]

I added it to avoid having to check isGpgCheck() for null every time, but I think it just made it more confusing, I'm happy to remove it.

[jmesnil]

+1, let's store the field as a boolean and deal only once with the nullability of the Boolean param in the constructor.

[spyrkob]

I ended up keeping the field as a Boolean to allow channels without gpg-check fields to be deserialized & serialized without changes. The public getter deals with the null check

[spyrkob]

@jmesnil this should be ready for re-review, sorry about the delay. Again if it's easier I can break this down into smaller PRs (e.g. API/Spec changes vs implementation)

[spyrkob]

@jmesnil would you be able to re-review the changes? I think it'd be good to get at least the channel part of this merged for WF35 so we don't have the problem with potential schema changes

[jmesnil]

@spyrkob This looks good apart from a few edits. Ping me when you make the change and I'll merge and release a new version. Thanks!

[spyrkob]

@jmesnil I addressed the review notes apart from the class name as it's part of bouncycastle library