Add Okta Oauth provider

[cmcculloh-kr]

Adds a provider for Okta. Can be used using:

new OktaOAuth2Provider({
    oktaDomain: import.meta.env.VITE_OKTA_DOMAIN as string,
    clientId: import.meta.env.VITE_OKTA_CLIENTID as string,
    clientSecret: import.meta.env.VITE_OKTA_CLIENTSECRET as string
})

Supersedes #56

[Dan6erbond]

Hey! These looks great! I'll merge it after some testing. Thanks for the PR.