draft: enable JIT compilation of RAJA kernels using Proteus

[johnbowen42]

This PR is a draft demonstrating integration of JIT compilation with Proteus into RAJA. Here are a couple of things to note about this PR:

• The CMake changes only support using Proteus via an installation, which must be provided to RAJA at build configuration time
• Depending on the configuration of the install of Proteus, it will be necessary to have LLVM shared libraries visible to the linker
• We can configure Proteus to be an optional submodule of RAJA as well, similar to CHAI/camp/umpire
• The current setup enables JIT at build config time. This means by default, with Proteus enabled, all RAJA kernel/launch/forall calls will be JIT compiled. We want to iterate on this by exposing JIT as an option that can be specified by the user