Allow the user-agent to be overridden

[msailes]

Allow the user-agent to be overridden by 3rd party developers of the runtime-interface-client.

Issue #, if available:
None

Description of changes:
When the aws-lambda-java-runtime-interface-client makes requests to the Lambda Runtime API it does them with a specific user-agent. This allows tracking and analysis of the clients and their versions.

Now that the aws-lambda-java-runtime-interface-client is being used by 3rd party framework developers it would be advantageous to be able to differentiate them.

This change introduces two additional ways that the user-agent can be overridden from the default. The original implementation sends 3 strings in the following format '%s/%s-%s'.

String userAgent = String.format(
        "aws-lambda-java/%s-%s" ,
        System.getProperty("java.vendor.version"),
        NativeClient.class.getPackage().getImplementationVersion());

The proposal allows for an override of the first string 'aws-lambda-java' if the env var 'RUNTIME_CLIENT' is present. If it is not then the default of 'aws-lambda-java' is used.

An override of the third string which is the pom version id with the env var 'RUNTIME_CLIENT_VERSION' if it is present. If it is not then the original implementation is used.

This change will introduce no changes to the user-agent for typical usage. It will allow 3rd party framework developers to set environment variables in bootstrap scripts to override the user-agent.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.