feat(R client): read from DELPHI_EPIDATA_KEY

[dshemetov]

This should make CI integrations easier, by allowing us to set the env var to a secret. Also mimics behavior in epidatr.

Summary:

• read API key from DELPHI_EPIDATA_KEY env var by default, else None
  • don't disturb the existing read from "epidata.auth" R option

Prerequisites:

• Unless it is a documentation hotfix it should be merged against the dev branch
• Branch is up-to-date with the branch to be merged with, i.e. dev
• Build is successful
• Code is cleaned up and formatted

[sonarcloud]

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[krivard]

👍 seems fine but I want george's thoughts too

if we keep it, please update the corresponding docs

[melange396]

(note: im not really at all familiar with R test processes/procedures, but im presuming theyre something like the tests we use with the python code in this repository)

if the motivation is to be able to provide a secret API Key for CI tests, theres no need: those tests dont use production data or services, and we can insert any old "test" key into the local test database during the pre-test setups.

if using environment variables (for secrets) is a standard paradigm for R clients and/or packages, id say go for it, but that getOption() looks like it achieves that already.

[dshemetov]

Yea, I'm just mimicking the convention of other packages - the R installers, for example, automatically pull out your GITHUB_PAT from your env vars, and so does the aws.s3 R module. And even broader than R, the aws cli does the same, so I figured this is an ⭐ "industry-standard" ⭐ thing ™️

[dshemetov]

No need