[SUREFIRE-2054] Support pluggable starters

**[Christoph Läubrich](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=laeubi)** opened **[SUREFIRE-2054](https://issues.apache.org/jira/browse/SUREFIRE-2054?redirect=false)** and commented

Currently Surefire has two (internal) Starters:

Forked:
https://github.com/apache/maven-surefire/blob/daf717e65716e3e77951757596c11d83754338e2/maven-surefire-common/src/main/java/org/apache/maven/plugin/surefire/AbstractSurefireMojo.java#L2437

InProcess:
https://github.com/apache/maven-surefire/blob/daf717e65716e3e77951757596c11d83754338e2/maven-surefire-common/src/main/java/org/apache/maven/plugin/surefire/AbstractSurefireMojo.java#L2454

[Tycho](https://www.eclipse.org/tycho/sitedocs/tycho-surefire-plugin/plugin-info.html) (and [BND](https://github.com/bndtools/bnd/tree/master/maven/bnd-testing-maven-plugin)) both offer "surefire-like" plugins that internally use surefire/junit.

I think this is a bit wasted effort, as actually most parts of surefire (configuration and alike) could simply be reused, both plugins simply need a mean to start the process differently.

My idea would be to make the Starters plugable, like one can plug in different platforms. Lookin at both Forked/InProcess the share a lot of common parameters and the run methods only differ in passing the effectiveProperties.

So I think it should be possible to derive a common "SurefireStarter" interface that an extender has to implement togehter with a "SurefireStarterFactory" that is used to construct such a starter passing the necessary information (e.g. the AbstractSurefireMojo and selected providers and so on...).

Of course this would be more an SPI than a API and result in rather tightly coupled plugins, but could make all this more reusable.

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No further details from [SUREFIRE-2054](https://issues.apache.org/jira/browse/SUREFIRE-2054?redirect=false)

Start with the Forked and InProcess starter construction and run methods in AbstractSurefireMojo.java at the linked locations. Compare their shared parameters and differing effectiveProperties handling, then investigate how a SurefireStarter interface and factory could support external plugins such as Tycho and BND. Done means the starter mechanism can be extended without duplicating the existing Surefire configuration and execution logic.

Written by the indexing model from the issue text.