mutLabels from cli inputs

Hi Nextclade devs, The `mutLabels` which is specified in the pathogen configuration file is super handy for calling mutations of interest. It's currently 'hidden' feature (because of the `pathogen.json` requirement) of nextclade that I think should be exposed directly as a CLI input. example usage: ``` nextclade run \ --input-ref ref.fasta \ --input-annotation ref.gff3 \ --input-mutations mutations.json ``` Where `mutations.json` is in the same format as the `pathogen.json`: ``` "mutLabels": { "aaMutLabelMap": { "HA1:145": ["RBD"], "HA1:155": ["RBD"] } } ``` or anything that is suitable for nextclade internals. The usecase would be to use nextclade to align and extract mutations of interest from viruses without having to create a complete nextclade dataset. Thanks!