Library configuration checks

[nbarbettini]

To help avoid misconfiguration, this library should implement these checks against common errors: https://oktawiki.atlassian.net/wiki/spaces/PM/pages/552049922/Library+configuration+checks

For this library, these checks apply:

• Issuer
• Client ID
• Client secret

Additionally, make sure these placeholders are always used in documentation or samples:

• https://{yourOktaDomain} (not {yourOktaDomain}.com)
• {clientId}
• {clientSecret}