Process-level container runtime directive to support hybrid operation

[pinin4fjords]

New feature

Currently, Nextflow supports a process-level executor directive, which is useful for [hybrid executions](https://www.nextflow.io/docs/latest/aws.html#hybrid-workloads) in scenarios where Docker is used across all executors. However, this approach has limitations when mixing container runtimes, for example, using Singularity locally and Docker on AWS Batch.

To address this, could we introduce a process-level containerRuntime directive? This would allow explicit pairing of the executor and container runtime, enabling more flexible and robust hybrid execution workflows.

Use case

In hybrid execution scenarios, changing the executor for a process often requires switching the container runtime accordingly. Having a dedicated directive to specify the container runtime per process would streamline this configuration and avoid potential mismatches.

Suggested implementation

process {
    executor = 'singularity'
    singularity.enabled = true

    withName: 'someprocess' {
        executor = 'awsbatch'
        containerRuntime = 'docker'
    }
}

[pditommaso]

This is already possible when one of the executor uses container "native", for example

nextflow/plugins/nf-amazon/src/main/nextflow/cloud/aws/batch/AwsBatchExecutor.groovy

Lines 95 to 98 in bde9ef1

	@Override
	final boolean isContainerNative() {
	return true
	}