Add subworkflow for gridss somatic structural variant calling

[imsarath]

This PR adds bam_tumor_normal_somatic_structural_variant_calling_gridss, a subworkflow that calls somatic structural variants from a tumor/normal BAM pair. It chains the full GRIDSS workflow - optional targeted fragment extraction, per-sample preprocess, joint assembly and call, then gridss_somatic_filter. It emits the unfiltered joint VCF plus the all-somatic and high-confidence somatic call sets.

PR checklist

Closes #4498

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