[ECS] Setup alternative log drivers & allow FireLens configuration

[izeau]

Is your feature request related to a problem? Please describe.
I would like to use the AWS FireLens log driver to send logs to Datadog. Unfortunately there appears to be no way to set the required firelensConfiguration parameter, the logConfiguration.logDriver parameter is currently hardcoded to awslogs and the logConfiguration.options parameter assumes we are using the awslogs driver anyway.

Describe the solution you'd like
To avoid breaking compatibility, a new custom_logging parameter would be nice, with generic driver (string), options (map(string)) and secret_options (map(string)) properties, mirroring the underlying LogConfiguration data type.

Describe alternatives you've considered
I’m considering using Waypoint but this missing feature is annoying.

Explain any additional use-cases
None off the top of my head

Additional context
None at the time, maybe later