Add in OurDNA data in main RD workflow

[MattWellie]

Purpose

• Integrate OurDNA into Main RD workflow
• Even if we don't want that included for every run, it can be commented out. The rest of this PR is a cleanup/restructure of the existing seqr_loader/annotate_cohort components, which makes the code easier to read, and optimises use of checkpoints to speed things up

Proposed Changes

• Tidies up the main method by pulling the external HT annotations out into separate methods

  • one for VEP, ClinVar, Seqr_loader
  • one for OurDNA (optional)
  • one for gnomAD 4 (commented out)

• Removes a checkpoint which wasn't doing anything (immediately after reading)

• Moves the global annotations up and condenses into a single mt.annotate_globals event

• Condenses the variant_qc annotation into a single block instead of splitting across 2 calls

The diff here might look a bit dodgy, but I think it will result in an easier to read process