Implement oauth_token_cached()

[hadley]

Fixes #328

@fh-mthomson does this look like it will be adequate for your needs? I think your code would look something like this:

oauth_token <- function() {
  OAUTH_AUTHORIZE_ENDPOINT <- "https://<X>.okta.com/oauth2/<X>/v1/authorize"
  REDIRECT_URI <- "https://<X>"
  SCOPE <- "session:role-any"
  
  oauth_token_cached(
    client = client,
    flow = oauth_flow_auth_code,
    flow_params = list(
      auth_url = OAUTH_AUTHORIZE_ENDPOINT,
      scope = SCOPE,
      redirect_uri = REDIRECT_URI
    ),
    cache_disk = TRUE
  )
}

oauth_client <- function() {
  OAUTH_CLIENT_ID <- "<X>"
  OAUTH_TOKEN_ENDPOINT <- "https://<X>.okta.com/oauth2/<X>/v1/token"
  
  httr2::oauth_client(
    id = OAUTH_CLIENT_ID,
    token_url = OAUTH_TOKEN_ENDPOINT,
    name = "oauth_client"
  )
}

[fh-mthomson]

This is exactly what I was hoping for! Thank you!