Add new target for Cairo executable

[maciektr]

Problem

Should compile project with use of compile_executable_in_prepared_db API.
This target should create new json output file with Executable of a function compiled before.

The main crate should have exactly one function marked as #[executable] (this is enforced by the compiler).

This functionality will heavily rely on the API exposed by cairo-lang-execuable crate in Cairo repo.

Proposed Solution

1. Add new pre-defined TargetKind called executable.

2. Add new compiler to compilers dir, called executable.

• This should be similar to the lib compiler
• Should implement the Compiler trait
• Should declare the executable target kind (from point 1.) as it's target_kind.
• Should read target dir, compiler config and main crates from CompilationUnit (see)
• Should compile main crates with compile_executable_in_prepared_db
• Do not provide any path, diagnostic reporter can be found in compiler_config.diagnostics_reporter

3. Wrap compilation result into Executable struct with Executable::new

4. Serialize the executable struct into JSON with serde

5. Write the result of serialization to the target directory with .executable.json extension.

6. Add created compiler to the CompilerRepository so Scarb can find it when compiling targets.

7. Add a test case to check if your new compiler can be used by users. It can be added to build_targets.rs test file. When creating a test project with ProjectBuilder, add [[target.executable]] to manifest with manifest_extra. Your lib.cairo (can be set with lib_cairo) should define a Cairo function with #[executable] attribute. Check if files in output directory are created properly.

Notes

Try dividing this work into consecutive PRs.