feat: add CI for a scaled down version of a cancer WES pipeline

[tedil]

This PR adds a snakemake workflow that executes a snappy-pipeline (which in turn invokes snakemake, but, well…) to be run as as a CI target. The output is not validated yet, so for now this simply tests whether execution is successful or not.
The snappy-pipeline is a basic cancer_wes one with steps:

• ngs_mapping
• somatic_variant_calling
• somatic_variant_annotation
• somatic_variant_filtration

where ngs_mapping is done using bwa, variant calling is done with mutect2, variant annotation is done with vep, and filtration is done with bedtools.
The reference is restricted to chr12, the raw reads (from the TCRB dataset) are filtered down to those mapping to a specific region on chr12, and the vep cache is also restricted to the same region.

(It would probably make sense to restrict the trigger such that it only runs when any of the steps/wrappers used are changed, but I'll leave it set to run on PR and on push to main for now)

[github-actions]

• Please format your Python code with black: make black
• Please format your Snakemake code with snakefmt: make snakefmt
• Please organize your imports isorts: make isort
• Please ensure that your code passes flake8: make flake8

You can trigger all lints locally by running make lint

[tedil]

This currently uses a post-deploy script https://github.com/bihealth/snappy-pipeline/pull/499/files#diff-74b65b07424e0be9f2b93fb89d5f2918b63381841c6feb6612a63da6aa20288c to install snappy-pipeline, which is stupid because the whole point is to use the version of the branch that is modified/the PR is based on.
That needs to be changed ^^
Still uses the post deploy script, but uses the snappy-pipeline version of the current state (of the PR/branch)

[coveralls]

coverage: 85.764% (+0.02%) from 85.742%
when pulling 5ce7038 on end-to-end-ci
into 6993233 on main.