Add Okta Oauth provider

[CrispyBacon1999]

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
})

[Dan6erbond]

Nice! One thing - OktaProvider needs to have the provider and its types exported in src/providers/index.ts so that it can be imported as a standard provider.

[cmcculloh-kr]

@Dan6erbond I'm interested in using Okta as an Oauth provider, so I made your requested changes, but I have no idea how to get them into this PR. They're here: https://github.com/cormacmccarthy/sk-auth

[vhscom]

I'm interested in using Okta as an Oauth provider, so I made your requested changes, but I have no idea how to get them into this PR. They're here: https://github.com/cormacmccarthy/sk-auth

If you based your work off this branch, it's possible for the maintainer to cherry-pick your commit adding the requested work into this branch so it can be tested. If that's the case, please indicate the commits to be cherry-picked or consider opening another Pull Request which wraps up the work and can be used to supersede this one assuming @CrispyBacon1999 is no longer interested in the follow-up work.

[Dan6erbond]

Closing as #98 supersedes it.