combine compilation steps in a pipeline

[iritkatriel]

Make it possible to chain compiler_codegen, optimize_cfg, assemble_code_object such that each stage takes the previous stage's output and collectively they do the same as compile().

Linked PRs

• gh-104240: make _PyCompile_CodeGen support different compilation modes #104241
• gh-104240: return code unit metadata from codegen #104300

[hugovk]

Ready to close? Thanks!