V2 SDK S3TransferManager with SSL Factory

### Describe the issue

I'm migrating some V1 AWS SDK code to the V2 SDK. In V1, it was possible to define a custom SSL socket factory to use with the `TransferManager` (abbreviated):
```
ClientConfiguration cliConfig = new ClientConfiguration();
cliConfig.getApacheHttpClientConfig().setSslSocketFactory(new SdkTLSSocketFactory(...));
AmazonS3ClientBuilder clientBuilder = AmazonS3ClientBuilder.standard().withClientConfiguration(cliConfig)...;
TransferManagerBuilder.standard().withS3Client(clientBuilder.build())...
etc.
```
I don't see a way to do this with the V2 SDK. The new `S3ClientConfiguration` allows some other configurables but not the HTTP client, or the SSL factory. Similarly the `S3TransferManager` does not appear to expose these. This document - https://docs.aws.amazon.com/sdk-for-java/latest/developer-guide/http-configuration-netty.html - talks about "configuring the Netty-based HTTP client" but I don't see a way to get that client into the `S3TransferManager`.

Help is appreciated. Thank you.

### Steps to Reproduce

See snippet above

### Current behavior

It is possible with V1 to assign a custom SSL socket factory to the V1 transfer manager.

### AWS Java SDK version used

V1 to V2

### JDK version used

1.8

### Operating System and version

linux