Add new component: gridss/assemble

[imsarath]

As a part of the distributed computing GRIDSS subworkflow #4498 , I added this gridss/assemble module. This module wraps the assemble step, which performs genome-wide break-end assembly: it takes the per-sample preprocessed data (along with the input BAM/BAI files and reference genome) and assembles the reads supporting candidate structural variants into a single .sv.assembly.bam. This assembly is the input required for the downstream variant-calling step

PR checklist

Closes #4502.

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