feat: add composable `useOidc`

[trijpstra-fourlights]

I added a composable to the plugin.

While it's a preference, IMO it allows for slightly cleaner code while using this module. Furthermore, having it in allows the developer to choose how to interact with the module.

e.g.

const { login } = useOidc()
await login('/secure')

instead of

const { $oidc } = useNuxtApp()
await $oidc.login('/secure')

or

const { $oidc: { login } } = useNuxtApp()
await login('/secure')

[aborn]

Nice work, thanks a lot!