Codegen select fuzzer v1

[PaulCher]

Changelog category (leave one):

• Testing Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):
Implemented structure-aware fuzzing approach in ClickHouse for select statement parser.

Detailed description / Documentation draft:
Structure-aware fuzzing has been previously been used in many projects, where it has demonstrated its effectiveness. This approach was observed in V8 fuzzing by Google and implemented for ClickHouse parser fuzzing. The idea behind codegen_fuzzer is very simple: with the help of libfuzzer and libprotobuf-mutator we can fuzz language parsers with coverage. Technically, it is a grammar based fuzzer, where grammar is simply TOKEN*, but it is done with feedback, which would be very useful for finding interesting paths without much manual work.

If this approach would show any results, I would like to add similar fuzzers for other parts of the project.

gen.py -- script to generate C++ string generation routines and .proto messages
clickhouse-template.g -- human-generated list of tokens
clickhouse.g -- full list of tokens generated by call to script update.sh.

By adding documentation, you'll allow users to try your new feature immediately, not when someone else will have time to document it later. Documentation is necessary for all features that affect user experience in any way. You can add brief documentation draft above, or add documentation right into your patch as Markdown files in docs folder.

If you are doing this for the first time, it's recommended to read the lightweight Contributing to ClickHouse Documentation guide first.

Information about CI checks: https://clickhouse.tech/docs/en/development/continuous-integration/

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