Switch frame-benchmarking to runtime APIs

[ggwpez]

Currently the ExtrinsicBuilder is passed as CLI argument to the subcommand, instead of being an API on the runtime.

Something like could be enough:

// runtime API:
pub trait BenchmarkExtrinsicBuilder {
	/// List all available extrinsics that can be build with `build_extrinsic`.
	fn list() -> Vec<BenchmarkExtrinsicInfo>;

	/// Try to build an extrinsic and return the encoded extrinsic and the cursor.
	///
	/// The extrinsic is assumed to be dispatchable within the current block.
	///
	/// Cursor can be used to generate multiple extrinsics, in case that the generation is not stateless. One example would be an account nonce. Returning `None` as the next cursor indicates that no further extrinsic can be generated. The benchmarking framework may treat this as error.
	fn build_extrinsic(info: &BenchmarkExtrinsicInfo, cursor: u32) -> (OpaqueExtrinsic, Option<u32>);
}

pub struct BenchmarkExtrinsicInfo {
	pub pallet: RuntimeString,
	pub extrinsic: RuntimeString,
}

Affected are the benchmark extrinsic and benchmark overhead commands.