AWS::Transfer::Server - HostKey

## AWS::Transfer::Server-HostKey

## 2. Scope of request
Currently cloudformation supports creating a transfer server, but it does not allow a host key to be provided. It should support specifying a host key much like AWS::Transfer::User supports [SshPublicKey](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-transfer-user-sshpublickey.html).

## 3. Expected behavior

```json
{
"Type" : "AWS::Transfer::Server",
"Properties" : {
"Certificate" : String,
"EndpointDetails" : EndpointDetails,
"EndpointType" : String,
"HostKey": String,
"IdentityProviderDetails" : IdentityProviderDetails,
"IdentityProviderType" : String,
"LoggingRole" : String,
"Protocols" : [ Protocol, ... ],
"Tags" : [ Tag, ... ]
}
}
```

Feel free to describe other desired and relevant implementation details.

## Suggested Specific Test Cases

* Common use case: pass HostKey parameter as a string, or as a !Ref
* Note this is a sensitive piece of information and will likely be stored in parameter store or secrets manager
* This can be changed, but will impact significant users. Perhaps a warning, as exists in the UI, would be advisable?

## 5. Helpful Links to speed up research and evaluation

* [AWS API Spec](https://docs.aws.amazon.com/transfer/latest/userguide/API_CreateServer.html) that supports the host key
* [Current cloudformation](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-transfer-server.html) documentation for transfer server

## Category: Migration & Transfer

Start with the linked AWS CreateServer API specification and the current AWS::Transfer::Server CloudFormation documentation. Confirm the HostKey property shape and behavior for literal and !Ref values, then add coverage for both cases and document any warning or replacement impact when the key changes.

Written by the indexing model from the issue text.