configurable function exports

[radkomih]

Currently, all exported functions are hard-coded in runtime.go. However, there is a need for a configurable approach to determine what should be exported and when.

Examples:

• building wasm runtimes for benchmarking, which exports additional functions, whereas this is not necessary for development or release builds.
• building wasm runtimes from templates with different consensus modules (aura, babe) configured.