Add the option for source export

[MatthiasHertel80]

Instead of exporting libs https://github.com/Arm-Examples/CMSIS-Executorch/tree/main/ai_layer/engine/lib the engine could export the necessary sources and minimal build options into https://github.com/Arm-Examples/CMSIS-Executorch/tree/main/ai_layer/engine/src , and create a different layer file to optionally use.

This enables:

• different compilers
• different optimization options
• debugging in library source
• different target CPUs in same project
• keep target options like logging in a csolution target configuration

Also analyze how build times can be similar to the library variant.