Deprecate the loose base64 decoding.

[anakinj]

Description

I first started by implementing the base64 methods within the gem (#578), but that did not really make sense in the end and there was already a bunch of direct references to the Base64 module.

This PR:

• Adds a dependency to the base64 gem and makes the dependency handling compatible with future Ruby versions.
• Warns about base64 payloads with invalid content (RFC 2045). Support for this will be dropped in later versions.

Checklist

Before the PR can be merged be sure the following are checked:

• There are tests for the fix or feature added/changed
• A description of the changes and a reference to the PR has been added to CHANGELOG.md. More details in the CONTRIBUTING.md