Add support for MSK event source

I'd like to have MSK as an event source. Currently I work around this by adding the required configuration during packaging with the `--merge-template` option:

```
{
"Resources": {
"Handler": {
"Properties": {
"Events": {
"KafkaConsumer": {
"Type": "MSK",
"Properties": {
"StartingPosition": "LATEST",
"Stream": "arn",
"Topics": [
"topic1"
]
}
}
},
"VpcConfig": {
"SubnetIds": [
"subnet"
],
"SecurityGroupIds": [
"security-group-id"
]
}
}
}
}
}
```

The SAM should contain

* An `Event` with type `"MSK"`
* A `VpcConfig` property with subnets and security groups

The `Event` could follow the same API as the other sources, but I wouldn't how to nicely configure the vpc settings. Its my understanding that its required deploy the function in the same VPC as the MSK cluster for the MSK trigger to work. Furthermore, the VPC needs to have VPC endpoints to the STS and Lambda services but that's not necessarily what Chalice should do.

I'm open to contributing myself towards this functionality. I've already forked the repo and will try to create a PR. Input and discussion is more than welcome.

Start by reviewing Chalice's existing event-source APIs and the current --merge-template packaging path described in the issue. The work is done when MSK events are supported with the requested event configuration and VpcConfig for subnets and security groups, with coverage for the generated configuration.

Written by the indexing model from the issue text.