Allow routes to override some HTTP config values

[joshdover]

In the legacy platform, some plugins override the default HTTP config on the hapi server on specific routes. For instance, this may be used to allow larger-than-normal request bodies for a particular route.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[afharo]

After some investigations, I've only seen the following overrides across the entire project:

• payload.output: 'stream' + payload.parse: false used in the console
• payload.output: 'stream' + payload.allow: 'multipart/form-data' in the import endpoints
• payload.allow: 'application/json' + payload.maxBytes: N in the batch functions endpoint (/fns)

The allow options are only enforcing that the endpoint only accepts that. When that property is omitted, those Content-Types are still allowed but others as well. So I guess it makes sense to expose it as well.

Other options in the payload are: https://hapi.dev/api/?v=18.4.0#-routeoptionspayload

So far I'm only going to expose the already used ones. We can come back to this later if we feel like it.