[MNG-7484] Collect warnings from different MOJO executions and give a comprehensive summary at the end of the build

**[Christoph Läubrich](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=laeubi)** opened **[MNG-7484](https://issues.apache.org/jira/browse/MNG-7484?redirect=false)** and commented

In certain situations there are some useful warnings a mojo emits (e.g. missing encoding, deprecated usage of options, possible side-effects,...).

Especial with a tycho-build it is common to have many hundred of reactor projects in a build and one problem we see is, that these warnings are easily overseen and people are then confused/surprised.

On the other hand, if a build **fails** then there we see the error in the mojo and a summary at the end of the build.

My idea would be, to have something similar for warnings, so a mojo can add a project specific warning that is summarized at the end. At best, it would even summarize duplicate warnings.

One idea would be to have an option to add to the BuildSummary warnings that can then be used to the CLI to produce such a summary.

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No further details from [MNG-7484](https://issues.apache.org/jira/browse/MNG-7484?redirect=false)

Start by reading the BuildSummary concept mentioned in the issue and tracing how the CLI currently reports build failures. Determine how mojo-specific warnings could be collected across reactor projects and summarized at the end, including duplicate warnings; completion would require an agreed design and a comprehensive warning summary.

Written by the indexing model from the issue text.