[MBUILDCACHE-33] Support remote cache credentials from environment variables

**[Alexander Ashitkin](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=alex_ashitkin)** opened **[MBUILDCACHE-33](https://issues.apache.org/jira/browse/MBUILDCACHE-33?redirect=false)** and commented

In my current environment settings.xml are managed by a build team which is not allowing any modification because the same build service is used by all teams. Atop of that, maven build runs in a fresh container which doesn't have any credentials injected for security reasons. Because of that cache cannot read/deploy build artifacts to an authenticated http server. Still, our build service allows to inject credentials from environment variables into build container. Need to support cache setup without settings.xml by injecting environment variables:
* MAVEN_BUILD_CACHE_DIRECT_CONNECT
* MAVEN_BUILD_CACHE_USER
* MAVEN_BUILD_CACHE_PASSWORD
* MAVEN_BUILD_CACHE_PROXY_USER
* MAVEN_BUILD_CACHE_PROXY_PASSWORD

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**Remote Links:**
- [GitHub Pull Request #35
](https://github.com/apache/maven-build-cache-extension/pull/35)

Start by reviewing the requested MAVEN_BUILD_CACHE_* environment variables, the existing settings.xml constraint, and the linked GitHub Pull Request #35. Done means cache setup can obtain direct-connection and server or proxy credentials from the specified environment variables without requiring settings.xml.

Written by the indexing model from the issue text.