spackbot DCO signoff

[kwryankrattiger]

Make it possible for spackbot to do DCO signoff on behalf of a user.

Two modes should be possible

• signoff
• signoff squash

The default mode would iterate commits, and if the commit is authored or co-authored by the user update the message with their signoff.

In the case that is not possible return an error and recommend using signoff squash

signoff squash should iterate the non-merge commits and collect all commit authors and valid signoffs. Then squash all of the commits into a single commit authored by the signer and reapply the other authors (not the signer) as co-authors. All of the existing signoffs, and the signers signoff, should then be applied uniquely to the squashed commit.

On DCO check failure, post information about how to do the signoff either with spackbot or manually using git.