Add support for custom icons as pipeline logos

[robnewman]

In parallel with the ongoing effort for supporting custom icons for pipeline logos in Seqera Platform, the CLI also needs to support this planned net-new functionality.

As a bioinformatician adding a pipeline programmatically, 
I want to define a custom icon as a pipeline logo, 
so that I visually identify my, or my organizations', pipelines.

Order of precedence:

1. Custom icon attached via URL (uploaded to platform)
2. Defined in Nextflow nextflow.config manifest.icon value
3. GitHub/BitBucket/GitLab organization icon
4. Auto-generated placeholder icon

[pditommaso]

Not sure how this feature would need CLI support