notifications: add support for recipients list based on architecture

Today we have a [recipient list](https://github.com/kernelci/dashboard/blob/main/backend/kernelCI_app/management/commands/data/recipients.yaml) based on trees. We want o add an `arch:` section, so eg RISC-V people can subscribe to riscv build and test issues.

Start with backend/kernelCI_app/management/commands/data/recipients.yaml to understand the existing tree-based recipient list, then trace where that configuration is read for notifications. Add the architecture-based recipient section described in the issue, and verify that subscribers to an architecture such as RISC-V receive the relevant build and test notifications.

Written by the indexing model from the issue text.