Feature request: ability to filter by compiler (build env) in email reports

[gctucker]

Build reports currently include all the builds for a given kernel revision on a branch. This starts to be come an issue as the number of compilers increases, in particular when the audience for some compiler versions is much smaller than the regular mailing lists. For example, build report emails with old LTS or new development compiler versions should be sent to a subset of recipients.

Doing this would require adding an optional field to the send API entry point to filter the build environments to include in the email report. The list of build environments to include in each report can be derived from the build-configs.yaml file on the client side as this is where the list of builds is coming from in the first place.