Postgres implementation of load_seed macro

Describe your changes

Implements a AWS RDS-centric implementation of a postgres load_seed macro implementation.

This relies on two optional variables set in your dbt project to override the S3 bucket name and optionally provide a prefix.

This assumes you've created a pg-friendly format of the seed data (this repo can be used to generate one). This macro implementation also requires that you have setup your RDS cluster/instance with a IAM Role that has the proper privileges to access the S3 bucket where the seed data is stored.

Also fixed PR template style guide link.

How has this been tested?

We've run this a few times using dbt cloud in individual and our production environments. We have not however done exhaustive testing around data quality.

Reviewer focus

This implementation is AWS+RDS specific, so it is unlikely to work for a postgres instance that is not hosted via AWS' RDS offering. There may be other pg extensions to read data from S3, but those have not been explored or tested.

Checklist before requesting a review

[X] I have added at least one Github label to this PR (bug, enhancement, breaking change,...)
[X] My code follows style guidelines
[ ] (New models) YAML files are categorized by sub folder and models listed in alphabetical order
[ ] (New models) I have added a config to each new model to enable it for claims and/or clinical data
[ ] (New models) I have added the variable tuva_last_run to the final output
[ ] (Optional) I have recorded a Loom to explain this PR

Package release checklist

[ ] I have updated dbt docs
[ ] I have updated the version number in the dbt_project.yml

tuva-health / tuva