feat: add OpenID Connect (OIDC) provider

[larsrickert]

closes #25
closes #89

This PR adds support for a generic OpenID connect (OIDC) provider that can be used with any provider that supports the OIDC standard.

It supports the 'code' response type and grant type 'authorization_code'. If a client secret is provided, it will be used to fetch the token. Otherwise the PKCE flow will be used where no client secret is needed.

Since the existing PR #25 has not seen progress in almost two years, this PR is intended to replace #25.

Example usage

# .env
NUXT_OAUTH_OIDC_CLIENT_ID=your-client-id
NUXT_OAUTH_OIDC_OPENID_CONFIG=https://my-provider.com/.well-known/openid-configuration

// server/routes/auth/oidc.ts
export default defineOAuthOidcEventHandler({
  config: {
    scope: ['openid', 'profile', 'email'],
  },
  async onSuccess(event, { user }) {
    await setUserSession(event, {
      user: {
        oidc: user.name,
      },
      loggedInAt: Date.now(),
    })

    return sendRedirect(event, '/')
  },
})