Add support for send a file security features

[samuelhwilliams]

Adds client support for the new 'confirm email before download' and 'custom retention period' features when sending a file by email.

Bumps the client to v6.1.0

What problem does the pull request solve?

Checklist

• [x] I’ve used the pull request template
• [x] I’ve written unit tests for these changes
• [x] I’ve update the documentation (in DOCUMENATION.md and CHANGELOG.md)
• [x] I’ve bumped the version number (in src/GovukNotify/GovukNotify.csproj)

[samuelhwilliams]

Pulled in content from https://github.com/alphagov/notifications-net-client/pull/163