Add support for send a file security features

[samuelhwilliams]

Adds optional parameters confirm_email_before_download and retention_period to the prepare_upload helper, to access new security features when sending a file by email.

Bumps the client version to 5.4.0

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 updated the documentation (in DOCUMENTATION.md and CHANGELOG.md)
• [x] I’ve bumped the version number (const VERSION in src/Client.php)

[samuelhwilliams]

@idavidmcdonald just for the sake of having 6 PRs to handle, and even then just to keep changes isolated, I propose we merge this as-is and handle bumping PHP versions separately if we want/need to.

[samuelhwilliams]

Pulled in content from https://github.com/alphagov/notifications-php-client/pull/117