Adds caching of git heights. As calculating the height of repositories with large volumes of commits is expensive, caching the git heights can save time in the following circumstances:
- Repetitive invocations of the `GetBuildVersion` msbuild task- e.g. during `dotnet pack`'ing a project, the `GetBuildVersion` task is invoked four times
- Incremental versioning- in cases where a cached height is available for older commits, this value will be used avoid the cost of recalculating the entire git height (only new commits will need to be traversed)

The caching behavior is enabled by default but can be opted-out by setting the new `NerdbankGitVersioningUseHeightCache` msbuild property to `false`

## Testing
### Automated testing
- Verified `GitHeightCache` can serialize + deserialize heights correctly
- Verified caching has a measurable impact on performance for cases when there are many commits to traverse
- All existing tests pass

### Manual testing
Consumed locally-packed version of `Nerdbank.GitVersioning` in a C# project and added ~1500 commits, verifying:
- Height caching takes effect on second build, dramatically decreasing build time (~10s -> ~1s)
- Adding an additional single commit can leverage the cached version for the previous ~1500 commits but adds the latest
- Setting the `NerdbankGitVersioningUseHeightCache` property to `false` bypasses the caching behavior.

…esn't match

…the build along

…mmits with multiple parents (any cached height would be used and could prevent the relevant commit graph being walked)

…now cached on disk next to the `version.json`/ `version.txt` file in order that performance improvements can be seen by multiple projects.