GMail: Feature - New Send Email Action

[tapishj-splunk]

Pull Request Checklist

Please check if your PR fulfills the following requirements:

• Testing of all the changes has been performed (for bug fixes / features)
• [] The manual_readme_content.md has been reviewed and added / updated if needed (for bug fixes / features)
• Use the following format for the PR description: <App Name>: <PR Type> - <PR Description>
• Provide release notes as part of the PR submission which describe high level points about the changes for the upcoming GA release.
• Verify all checks are passing.
• Do NOT use the next branch of the forked repo. Create separate feature branch for raising the PR.
• Do NOT submit updates to dependencies unless it fixes an issue.

Pull Request Type

Please check the type of change your PR introduces:

• New App
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Documentation
• Other (please describe):

Security Considerations (REQUIRED)

• If you are exposing any endpoints using a REST handler,
  please document them in the manual_readme_content.md.
• If this is a new connector or you are adding new actions
  • Please document in the manual_readme_content.md all methods (eg, OAuth) used to authenticate
    with the service that the connector is integrating with.
  • If any actions are unable to run on SOAR Cloud, please document this in the manual_readme_content.md.
• Are you introducing any new cryptography modules? If yes, please elaborate their purpose:
• Are you are accessing the file system? If yes, please verify that you are only accessing paths returned through
  the Vault API.
• Are you are marking code to be ignored by Semgrep with nosemgrep?
  If yes, please provide justification in an additional comment next to the ignored code.

Release Notes (REQUIRED)

• New send email action that allows users to send emails. Provides the ability to include attachments, allows users to specify reply-to, bcc, cc and create/use custom aliases. Note to create an alias users do need to authorize the following scope: https://www.googleapis.com/auth/gmail.settings.sharing, which they currently don't need to do.

Screenshots (if relevant)

Email sent with attachment and cc:

Reply-to test:

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Thanks for contributing!