benchmark

Output library / schema versions in JSON context block (#1742)

* CMake: `get_git_version()`: just use `--dirty` flag of `git describe`

* CMake: move version normalization out of `get_git_version()`

Mainly, i want `get_git_version()` to return true version,
not something sanitized.

* JSON reporter: store library version and schema version in `context`

* Tools: discard inputs with unexpected `json_schema_version`

* Extract version string into `GetBenchmarkVersiom()`

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Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>