How to add http proxy event ?

Hello,
I'm trying to understand how I can handle an http proxy event from lambda using serverless-offline.

The configuration works fine on aws api gateway but I can't find how to make it work locally.

I have a server and can access images using direct address example: `http://localhost:9999/image1.png` (on aws it's on a s3 bucket with cloudfront address)
I would like this to be served by the proxy a request to http://localhost:3000/images/images1.png should serve the image http://localhost:9999/image1.png

Here is a simplified version of my configuration. Is there a way to make it works on serverless-offline currently ?
I have `resourceRoutes: true` as the documentation suggest but it doesn't seems to work. http://localhost:3000/images/images1.png returns nothing

```
service: myapp
frameworkVersion: "3"
useDotenv: true
variablesResolutionMode: 20210326
disabledDeprecations:
- CLI_OPTIONS_SCHEMA

provider:
name: aws
stackName: myapp
architecture: arm64
runtime: nodejs14.x
region: ${opt:region, 'us-west-2'}
stage: ${opt:stage, 'prod'}
logRetentionInDays: 7
lambdaHashingVersion: 20201221
versionFunctions: false
apiGateway:
binaryMediaTypes:
- '*/*'

environment:
NODE_ENV: ${self:custom.nodeenv.${self:provider.stage}}
SERVERLESS_PROJECT: ${self:service}
SERVERLESS_REGION: ${self:provider.region}
SERVERLESS_STAGE: ${self:provider.stage}

package:
individually: true

plugins:
- serverless-webpack
- serverless-plugin-scripts
- serverless-offline

functions:
myapp:
memorySize: 512
provisionedConcurrency: 0
timeout: 6
handler: handler.serve
name: myapp
events:
- http:
path: /images/{any+}
method: ANY
cors: true
integration: http-proxy
request:
uri: 'http://localhost:9999/{any}'
parameters:
paths:
any: true
- http:
path: /
method: any
cors: true

- http:
path: /{any+}
method: any
cors: true

custom:
webpack:
webpackConfig: "webpack.server.config.js"

serverless-offline:
resourceRoutes: true
useChildProcesses: false
noPrependStageInUrl: true
httpPort: 3000
lambdaPort: 3002

resources:
Resources:
ApiGatewayRestApi:
Type: AWS::ApiGateway::RestApi
Properties:
Name: myapp
```

Start with serverless-offline's resourceRoutes handling and the http-proxy event configuration for /images/{any+}. Reproduce the request to http://localhost:3000/images/images1.png against the upstream http://localhost:9999/image1.png endpoint; done means the local proxy serves the upstream image as configured.

Written by the indexing model from the issue text.