Implement solution based merging for data collector

VSTest allows to post process artifacts, now we can implement solution wide report merge.

RFC https://github.com/microsoft/vstest-docs/blob/main/RFCs/0031-Test-Run-Attachments-Processing.md#how-to-register-an-idatacollectorattachmentprocessor

Contributes to https://github.com/coverlet-coverage/coverlet/issues/357