[3.8.x][MNG-7400] - Allow more WorkspaceReader's to participate

[laeubi]

This is a backport of #667

we at Tycho have a high demand in integrating better with maven to reduce some workarounds that where used in the past to get custom dependency types into the maven dependency model (namely using system scoped absolute paths).

For that reason it would help if it where possible to register additional WorkspaceReader to participate.

I have created a prototype to show/test this new feature.

FYI @mickaelistria @akurtakov

This PR allows maven-extensions to supply WorkspaceReader as it is possible with AbstractMavenLifecycleParticipant currently it is only possible with some workarounds:

1. One needs to register a AbstractMavenLifecycleParticipant in a maven-core-extension
2. In the afterSessionStart cast the RepositorySystemSession to DefaultRepositorySystemSession
3. Call setWorkspaceReader and take care not to replace an already exiting one

With this change it is possible to simply register WorkspaceReader component directly and Maven takes care of all the rest.

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[laeubi]

@gnodet thanks for reviewing this, I found a small issue that the ReactorReader was added twice to the list, I fixed this now.

[laeubi]

@gnodet anything left here to do until it could be merged?

[gnodet]

@michael-o is the release manager for 3.8.x branch, so I'll defer to him

[michael-o]

Please synchronize with master.

[michael-o]

Merged.