Using for 3rd party tenants

#### Support plan

* *which support plan is this issue covered by?*: Community
* *is this issue currently blocking your project?*: yes
* *is this issue affecting a production system?*: no

#### Context

* *node version*: 12.18
* *module version*: 19.1
* *environment* (e.g. node, browser, native): node
* *used with* (e.g. hapi application, another framework, standalone, ...): hapi application
* *any other relevant information*: Tried with Twitter, but same for all

#### How can we help?

We would like to allow our users to build out social logins custom to their project. This means that, within each project and for each social provider, there would be a different `clientId` and `clientSecret`.

Example:
Project A, Twitter login:
`clientId: aaaaaaaaaaaaaaaa`
`clientSecret: aaaaaaaaAAAAAAAAaaaaaaaAAAAAAA`

Project B, Twitter login:
`clientId: bbbbbbbbbbbbbbbb`
`clientSecret: bbbbbbbBBBBBBBBBbbbbbbbBBBBBBB`

However the documented method is to configure for a single application for each provider and then register a route handler:

Registration
```js
async function registerSocial() {
await server.register(Bell)

server.auth.strategy('twitter', 'bell', {
provider: 'twitter',
password: 'cookie_encryption_password_secure',
clientId: 'aaaaaaaaaaaaaaaa',
clientSecret: 'aaaaaaaaAAAAAAAAaaaaaaaAAAAAAA',
})
}
```

Route handler
```js
{
method: ['GET', 'POST'],
path: '/auth/twitter/login',
handler: async (request, h) => {
try {
if (!request.auth.isAuthenticated) {
return `Authentication failed due to: ${request.auth.error.message}`
}

// Perform any account lookup or registration, setup local session,
// and redirect to the application. The third-party credentials are
// stored in request.auth.credentials. Any query parameters from
// the initial request are passed back via request.auth.credentials.query.

return h.redirect('/')
} catch (err) {
return routeUtils.handleErr(err, h)
}
},
config: {
auth: {
mode: 'try',
strategy: 'twitter',
},
},
},
```

Is it possible to perform the Registration step in some other way? Potential solutions might include:

- Register strategy generically, and pass in `clientId` / `clientSecret` another way
- Register strategy with dummy `clientId` / `clientSecret` and change later in route handler
- Look up `clientId` and `clientSecret` with a method in the options object of the registration, based on a project id in the URL
- Look up `clientId` and `clientSecret` as a method in the `config` object of the route config, based on a project id in the URL

If this is supported, we would love a hint at the documentation. If it is not supported, we would love a nudge in the right direction for how we might implement this ourselves.

Thank you!