[MRELEASE-346] Refactor MavenExecutor out into some shared component (e.g. Invoker)

**[Benjamin Bentmann](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=bentmann)** opened **[MRELEASE-346](https://issues.apache.org/jira/browse/MRELEASE-346?redirect=false)** and commented

The code from the package `org.apache.maven.shared.release.exec` is not very specific to the release job but basically duplicates functionality from the maven-invoker. Ideally this stuff would merge someday.

See: https://github.com/apache/maven-release/tree/maven-release-3.0.0-M2/maven-release-manager/src/main/java/org/apache/maven/shared/release/exec 

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**Affects:** 2.0-beta-7

**Issue Links:**
- [MRELEASE-905](https://issues.apache.org/jira/browse/MRELEASE-905) Provide better documentation on mavenExecutorId parameter

- [MSHARED-20](https://issues.apache.org/jira/browse/MSHARED-20) Extend Invoker API to support the needs of the Release Plugin
(_**"depends upon"**_)

Start by reviewing the classes under maven-release-manager/src/main/java/org/apache/maven/shared/release/exec and compare their functionality with the maven-invoker. Read MSHARED-20 to understand the required Invoker API support. Done means the release-specific executor functionality is provided by a shared Invoker component without duplicating maven-invoker behavior.

Written by the indexing model from the issue text.