proxy issue - maven central URL

Hi,

I would like to install `localstack[full]` with pip , but I have an Internet proxy.

I have set my pip entreprise proxy (nexus) in my `pip.ini` file for "indexes".
And I can't set HTTP(S)_PROXY env variables otherwise `pip install` doesn't find any package because my nexus proxy is not found on Internet.

The installation of `amazon-kclpy-ext` - a localstack's dependency - failed: JAR files can't be downloaded:
https://github.com/awslabs/amazon-kinesis-client-python/blob/5dfa68e2a654373aed122476526905a737327721/setup.py#L160-L162
Error:
```
Attempting to retrieve remote jar https://search.maven.org/remotecontent?filepath=com/amazonaws/amazon-kinesis-client/1.9.3/amazon-kinesis-client-1.9.3.jar
Failed to retrieve https://search.maven.org/remotecontent?filepath=com/amazonaws/amazon-kinesis-client/1.9.3/amazon-kinesis-client-1.9.3.jar:
```

Is there any way to configure `prefix`?