Add plink_gwas_regenie subworkflow

[lyh970817]

Description

Adds the plink_gwas_regenie subworkflow to run REGENIE Step 1 model fitting followed by Step 2 association testing from PLINK-format genotype bundles.

This subworkflow composes plink_fit_regenie with regenie/step2, supports standard and chunked Step 1 execution, and supports multiple Step 2 genotype shards sharing the same analysis meta.id.

Draft dependencies: this PR depends on #11768 for regenie/runl1 and #11937 for plink_fit_regenie.

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
• Remove all TODO statements.
• Broadcast software version numbers to topic: versions - See version_topics
• Follow the naming conventions.
• Follow the parameters requirements.
• Follow the input/output options guidelines.
• Ensure that the test works with either Docker / Singularity. Conda CI tests can be quite flaky:
  • For subworkflows:
    • nf-core subworkflows test plink_gwas_regenie --profile docker --dir .
    • nf-core subworkflows test plink_gwas_regenie --profile singularity
    • nf-core subworkflows test plink_gwas_regenie --profile conda