Add support for proxy for cli dependency resolution

[hpmellema]

resolves #2029

Description of changes:
Adds support for a proxy configuration for maven repositories.

This proxy configuration can be added one of 2 ways:

1. Via environment variables: SMITHY_PROXY_HOST and SMITHY_PROXY_CREDENTIALS . These will set a common proxy configuration for all repositories
2. Via a specific configuration on the Maven repository definition. For example,

    "maven": {
        "repositories": [
            {
                "url": "http://localhost:1234/maven/not/there",
                "proxyHost": "http://localhost:2323",
                "proxyCredentials": "user:pass"
            }
        ],
        "dependencies": [
            "software.amazon.smithy:smithy-aws-iam-traits:1.26.0"
        ]
    }

The per-repo configuration of the proxy will override any settings set by the environment variables.

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

[mtdowling]

I think we've been using a SMITHY_ prefix for our owned environment variables. Is there a reason to not prefix PROXY_HOST and PROXY_CRED?