Add console runtime for Symfony Console and Laravel Artisan

[mnapoli]

Closes #120

I implemented this as a thin layer that comes on top of the PHP runtime so that it's much easier to maintain and not coupled to a specific PHP version.

To use this: include the PHP runtime and add the "console" layer on top of it.

Usage example:

Resources:
     MyFunction:
         Type: AWS::Serverless::Function
         Properties:
             [...]
             Handler: bin/console
             Layers:
                 - '<PHP layer ARN here>'
                 - '<console layer ARN here>'

Then to execute a command:

vendor/bin/bref cli <function name> -- doctrine:migrate

TODO:

• runtime
• bref cli command
• publish runtime

[mnapoli]

I created a follow up in #125 to simplify the use of the bref cli command.