✨ Add DRS implementation

[bwalsh]

This PR adds support for DRS, addresses #23.

There are several topics raised on #23, this PR addresses the basic blocking and tackling:

• bottom-up: extension of Attachment to include DRSMeta, DRSChecksum and DRSAccessMethod
• top-down: example ResearchStudy->ResearchSubject->ResearchSpecimen->Task->DocumentReference.DRSMeta
• middle: Suggest a separate PR(s) to address the issue raised on kfdrc-genomic-file and kfdrc-sequencing-experiment

Current implementation in an AnVIL environment here

[liberaliscomputing]

@bwalsh, whenever there are changes in resource entries, please run fhirutil ./site_root/input/resources so that those changes can be reflected in the IG config file.

[liberaliscomputing]

@bwalsh, all the changes you've made look good! Before approving this, let's clean up the commit history as follows:

• You would probably have 8 commits on your local branch (check this by git log). Once you've confirmed the number of commits, please reset your commits by git reset HEAD~[n] where [n] is the actual number of commits (e.g. git reset HEAD~8).
• Group the staged files in meaningful segments. I think we can create two commits: 1) updating .gitignore and 2) adding resources. Please note that we use gitmoji to visually/intuitively tag commit messages as follows,
  • git commit -m ":wrench: include .DS_Store"
  • git commit -m ":sparkles: profiles DRS resources"
• Once committing is done, do git push -f origin [your-branch-name] which will overwrite the previous pushes.