feat: Adds an option to make DKIM validation optional

[qhenkart]

Fixes

Issues seem to have been disabled for this Repo so I could not make one

Allows the inbound email SDK to support more email clients depending on the security requirements of the service. It also lays infrastructure for adding more functionality and configuration

A short description of what this PR does.

Simply takes an optional configuration struct that allows DKIM to be set to optional, so it will still check for failures but it will allow email clients that don't have DKIM set up

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [x] I have added tests that prove my fix is effective or that my feature works
• [n/a] I have added the necessary documentation about the functionality in the appropriate .md file
• [x] I have added inline documentation to the code I modified

If you have questions, please file a support ticket.

[qhenkart]

Looks like the build/test scripts are broken, unrelated to the update in this PR.

[qhenkart]

@shrutiburman Is this repo still actively maintained?