Add proxy configuration support

[conniey]

Adds support for proxy configuration:

• Users can specify the proxy hostname, authentication type, username and password.
  • If they do not, the system defaults are used which the ProxyConfiguration.SYSTEM_DEFAULTS constant. (This is the existing behaviour.)
• There are two configuration mismatch scenarios that will fail to connect:
  • If user specifies an authentication type that is not supported.
  • If user specifies ProxyAuthenticationType.BASIC or ProxyAuthenticationType.DIGEST and authentication is not required.
• Adds unit/integration tests to verify that the proxy configuration is used.

Miscellaneous changes:

• Add slf4j-simple dependency for test scope (to see logging output).
• Enhances and adds unit/integration tests for existing Digest and Basic ChallengeProcessors to ensure that the returned authentication values are what we expect.

TODO:

• [x] Verify that this works with squid proxy.

[conniey]

@JamesBirdsall I updated it. How does it look now? 🤔

[conniey]

@JamesBirdsall I can't merge this PR. :( I'm not an authorized user. Would you mind merging it? Thanks! I tested it on the squid proxy and it works! :)

[Edit]: Also, I have another PR after this.. unrelated to Proxy configuration, but DatatypeConverter is removed in JDK 11, so I wanted to make this future proof by replacing it.