Add GetCompiledCasm endpoint

[ArielElp]

Today, all nodes store the compiled CASM artifacts but do not externalize them. Third parties can use them and save the overhead of compilation, concrete usecases are:

• Replay startknet via a json-rpc based state reader, without the new endpoint replay time & complexity may be dominated by compilation
• Running the Starknet OS requires access to compiled CASM (this usecase also requires the bytecode_segment_lengths data, which is not stored ATM on nodes, hence it remains optional on the spec)

In this PR we add a starknet_executables.json section to the API, with a new endpoint to return the compiled CASM. The specification attempts to reflect the json serialization of CasmContractClass, and should agree with the feeder gateway's get_compiled_class_by_class_hash endpoint.

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This change is 

[ArielElp]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ArielElp)