feat: Implement IEquatable in EmailAddress

[aevitas]

Fixes #425

This pull request enables use of Distinct() et. al. on email addresses, allowing users to easily ensure no duplicate recipients exist, for instance when pulling recipients from a data source.

Checklist

• I acknowledge that all my contributions will be made under the project's license
• 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 development branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Implement IEquatable<EmailAddress> in EmailAddress
• Allows use of LINQ methods such as Distinct() on lists of e-mail addresses

[aevitas]

I just realised that this PR lacks the implementation of the equality operators as per the IEquatable documentation

[aevitas]

I just realised that this PR lacks the implementation of the equality operators as per the IEquatable documentation

I've just finished implementing these. @thinkingserious PTAL.

[RodgerLeblanc]

Looks good to me

[childish-sambino]

Fixed the stack overflow issue and equality logic to only check the email address and make it case-insensitive per comment here: #425 (comment)