Make DCO check more developer friendly

[samikshya-db]

DCO tests in GitHub Actions ensure code contributions are legally sound. We are implementing stricter DCO checks. Currently, DCO compliance is not enforced as a pre-merge requirement.

Sample PR where DCO passes : https://github.com/databricks/databricks-sql-python/pull/461/commits.

But, sadly the sign-off should be part of every commit message for DCO GH action to pass. The idea is to make it more developer friendly.

[dhirschfeld]

I suppose it's theoretically possible to rewrite history to add a Signed-off-by to a merge commit, but I don't think that really makes much sense and it would be better for the check to ignore merge commits.

[dhirschfeld]

The DCO check does appear to be broken though:

• DCO check is broken #520