add helpers for secret env vars

[maxheld83]

when in CI, these should be read in from (secret) env vars, when local, these should come from the OS secret store (i.e. keychain).

See:

• https://github.com/maxheld83/passhelpr
• https://httr.r-lib.org/articles/secrets.html
• https://gargle.r-lib.org/articles/articles/managing-tokens-securely.html
• ... and probably more in devtools or usethis?