customAuthenticationProvider breaks if the service has more than one function

## Bug Report

I'd be interested in working on a pull request for this fix, if the maintainers are open to it, I just need some help understanding the intent of the name/scheme properties in the custom auth provider.

**Current Behavior**

customAuthenticationProvider only works if the service has a single function

`HttpServer.setAuthorizationStrategy` loops over every function event and calls `auth.scheme()` on the custom auth provider, but `auth.scheme()` throws if you try to add the same scheme multiple times.

**Sample Code**

https://github.com/humodz/serverless-offline/blob/master/tests/integration/custom-authentication/serverless.yml

- file: serverless.yml

```yaml
service: integration-tests

configValidationMode: error
deprecationNotificationMode: error

custom:
offline:
customAuthenticationProvider: ./src/authenticationProvider

plugins:
- ../../../src/index.js

provider:
architecture: arm64
deploymentMethod: direct
memorySize: 1024
name: aws
region: us-east-1
runtime: nodejs18.x
stage: dev
versionFunctions: false

functions:
echo:
events:
- httpApi:
method: get
path: /echo
handler: src/handler.echo
echo2:
events:
- httpApi:
method: get
path: /echo2
handler: src/handler.echo
```

**Expected behavior/code**

customAuthenticationProvider should still work if the service has more than one function

**Environment**

- `serverless` version: 3.38.0
- `serverless-offline` version: 13.6.0 - just reproduced the issue with master@5139c43fc
- `node.js` version: v18.18.0
- `OS`: Ubuntu 20.04

**Possible Solution**

Change `HttpServer.setAuthorizationStrategy` or `auth.js : scheme()` so that it either allows adding the same scheme multiple times, or so that it stops adding the same scheme over and over for each function

**Additional context/Screenshots**

Stack trace:

```js
Error:
AssertError: Authentication scheme name already exists: scheme
at module.exports.internals.Auth.scheme (/home/hugo/projects/personal/serverless-offline/node_modules/@hapi/hapi/lib/auth.js:49:14)
at #setAuthorizationStrategy (file:///home/hugo/projects/personal/serverless-offline/src/events/http/HttpServer.js:419:25)
at HttpServer.createRoutes (file:///home/hugo/projects/personal/serverless-offline/src/events/http/HttpServer.js:1027:60)
at #createEvent (file:///home/hugo/projects/personal/serverless-offline/src/events/http/Http.js:41:22)
at file:///home/hugo/projects/personal/serverless-offline/src/events/http/Http.js:46:24
at Array.forEach ()
at Http.create (file:///home/hugo/projects/personal/serverless-offline/src/events/http/Http.js:45:12)
at #createHttp (file:///home/hugo/projects/personal/serverless-offline/src/ServerlessOffline.js:192:16)
at async Promise.all (index 0)
at async ServerlessOffline.start (file:///home/hugo/projects/personal/serverless-offline/src/ServerlessOffline.js:98:5)
at async #startWithExplicitEnd (file:///home/hugo/projects/personal/serverless-offline/src/ServerlessOffline.js:154:5)
at async PluginManager.runHooks (/home/hugo/projects/personal/serverless-offline/node_modules/serverless/lib/classes/plugin-manager.js:530:9)
at async PluginManager.invoke (/home/hugo/projects/personal/serverless-offline/node_modules/serverless/lib/classes/plugin-manager.js:564:9)
at async PluginManager.run (/home/hugo/projects/personal/serverless-offline/node_modules/serverless/lib/classes/plugin-manager.js:604:7)
at async Serverless.run (/home/hugo/projects/personal/serverless-offline/node_modules/serverless/lib/serverless.js:179:5)
at async /home/hugo/projects/personal/serverless-offline/node_modules/serverless/scripts/serverless.js:819:9
```