Define a DataStructure MigrationSummary

[hf-kklein]

Define a data structure MigrationSummary that collects information about a migration run (using the MigrationStrategy from #15).
It shall be meaningful and summarize what happend from source to target.
Note that some information are already being logged, e.g. in the validation or the filter.

Originally posted by @hf-kklein in #15 (comment)

[hf-kklein]

please use attrs:

@attrs.define(auto_attribs=True, kw_only=True)
class MigrationSummary:
    attributes = attrs.field(...)
    go = attrs.field(...)
    here = attrs.field(...)