Add built-in fuzzing harnesses for security testing.

Add built-in fuzzing harnesses for security testing.

Add 12 libFuzzer-compatible fuzzing harnesses behind a new -Dfuzzing=true
meson option.  Each harness implements LLVMFuzzerTestOneInput() and can
also be built in standalone mode (reading from files) when no fuzzer
engine is detected.

Frontend targets (no backend dependencies):
  fuzz_json            - non-incremental JSON parser (pg_parse_json)
  fuzz_json_incremental - incremental/chunked JSON parser
  fuzz_conninfo        - libpq connection string parser (PQconninfoParse)
  fuzz_pglz            - PGLZ decompressor (pglz_decompress)
  fuzz_unescapebytea   - libpq bytea unescape (PQunescapeBytea)
  fuzz_b64decode       - base64 decoder (pg_b64_decode)
  fuzz_saslprep        - SASLprep normalization (pg_saslprep)
  fuzz_parsepgarray    - array literal parser (parsePGArray)
  fuzz_pgbench_expr    - pgbench expression parser (via Bison/Flex)

Backend targets (link against postgres_lib):
  fuzz_rawparser       - SQL raw parser (raw_parser)
  fuzz_regex           - regex engine (pg_regcomp/pg_regexec)
  fuzz_typeinput       - type input functions (numeric/date/timestamp/interval)