Add a way to review a full series from cover letter

It'd be nice to have a way to review the whole series in one go by replying to the cover letter. However, currently people can write something like:

For patches 1 to 3:
Reviewed-by: Damien Lespiau damien.lespiau@intel.com

but the series has 10 patches, so can't quite mark the whole series as reviewed just from seeing a r-b tag. There are a few ways to solve this, eg. add more specialised tags:

Series-Reviewed-by: Damien Lespiau damien.lespiau@intel.com
Series-Reviewed-by: Damien Lespiau damien.lespiau@intel.com

Or add a bit of language on top of a Reviewed-by tag to specify a range of patches:

Patches 1-3: Reviewed-by: Damien Lespiau damien.lespiau@intel.com
Series: Reviewed-by: Damien Lespiau damien.lespiau@intel.com