This small PR adds a default feature `schur` to control the
`SCHUR_SPECIALIZATIONS` option in Ceres. It remains on by default.
As described briefly in the
[documentation](http://ceres-solver.org/installation.html#options-controlling-ceres-configuration),
`SCHUR_SPECIALIZATIONS` enables some template specializations which
improve runtime performance of the `SPARSE_SCHUR` solver, but it
requires a large number of explicit instantiations and generates large
object files—large enough to require `/bigobj` in MSVC. This can cause
issues with some compilers particularly in constrained environments.
- [x] Changes comply with the [maintainer
guide](https://github.com/microsoft/vcpkg-docs/blob/main/vcpkg/contributing/maintainer-guide.md).
- [ ] ~~SHA512s are updated for each updated download.~~
- [ ] ~~The "supports" clause reflects platforms that may be fixed by
this new version.~~
- [ ] ~~Any fixed [CI
baseline](https://github.com/microsoft/vcpkg/blob/master/scripts/ci.baseline.txt)
entries are removed from that file.~~
- [ ] ~~Any patches that are no longer applied are deleted from the
port's directory.~~
- [x] The version database is fixed by rerunning `./vcpkg x-add-version
--all` and committing the result.
- [x] Only one version is added to each modified port's versions file.
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