chore: add Rubocop configuration file

[RolandBurrows]

Short description of what this PR does:

1. Running rubocop locally per the contributing guide resulted in the following:

   87 files inspected, 2736 offenses detected

   But the vast majority of those offense are located in folders excluded by the .codeclimate.yml for CI. Excluding those locally yields:

   40 files inspected, 293 offenses detected

2. Note: that if someone runs rubocop -a to autocorrect issues:

   40 files inspected, 309 offenses detected, 236 offenses corrected

   but this touches 31 files and is likely to result in massive merge conflicts. Best to do after most major feature work is done.

3. Note: one can run rubocop --auto-gen-config and it will generate a .rubocop_todo.yml file that documents all the spots that need repairing. But it will also change the .rubocop.yml to inherit the todo file, which then excludes the issues from being reported. Best not commit that.

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• (N/A) I have added tests that prove my fix is effective or that my feature works
• (N/A) I have added necessary documentation about the functionality in the appropriate .md file
• (N/A) I have added in line documentation to the code I modified

[SendGridDX]

All committers have signed the CLA.