Add optional extraParams to clientCredentials

[South-Paw]

• Adds extraParams args to the clientCredentials method to support vendor-specific params
• Fixes some typos in code documentation

Fixes #90

[South-Paw]

appreciate all the spelling fixed too =)

no worries, I have a spell checker plugin in VSCode cause I disliked finding out I'd misspelling a name/fn and then proliferated it everywhere for weeks 😆

[South-Paw]

So in lieu of adding the resource param, I've removed it from the disallowed list as well