Update the processors to consolidate MRs based on file path

What does this MR do and why?

Describe in detail what your merge request does and why.

Based on feedback received on the update-test-meta tool, this MR consolidates MRs created by processors based on file paths to reduce review throughput.

The high level changes in this MR are:

1. Replace execute method in the processors with process_commit and process_merge_requests methods. The process_commit method has been placed in the MetaProcessor super class.
2. The process_commit writes the processed commits to processed_commits hash in the context (TestMetaUpdater). See this comment for the structure of the hash.
3. Once commits are processed, the process_merge_requests in each processor uses the processed_commits hash to create MRs.

Addresses #56 (closed)

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Please follow the same steps as in this MR: !117 (merged) except for the files passed to --stable_specs_file and --unstable_specs_file switches, use these files instead:

• stable_specs_multiple_per_file.json
• unstable_specs_multiple_per_file.json

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.