Move `http` event authorizer settings to top level `provider` section

Currently, the settings for a custom authorizer function are set within the event settings for each function:

functions:
  newFunction:
    handler: function.handler
    events:
      -  http:
            path: url/path
            method: get
            authorizer:
              name: customAuth
              resultTtlInSeconds: 30
              identityValidationExpression: .*

  customAuth:
    handler: auth.handler
    memorySize: 128

This leaves room for those settings to be redefined multiple times in the configuration, and can introduce uncertainty, particularly when there are multiple hands touching the code.

It would be beneficial to have everything related to the custom authorizer in one spot, and just reference the authorizer by name in each function. Something like this:

functions:
  newFunction:
    handler: function.handler
    events:
      -  http:
            path: url/path
            method: get
            authorizer: customAuth

  customAuth:
    handler: auth.handler
    memorySize: 128
    authorizerSettings:
      resultTtlInSeconds: 30
      identityValidationExpression: .*

---

Implementation proposal

Introduce similar authorizer configuration as it's for httpApi events. Individual authorizers should be configured at provider.apiGateway.authorizer settings and which are referenced at http events.

This new setup should be introduced in non breaking way, but with configured deprecation of authorizer configurations at http event level