pgsql: 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)

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/4a18907b412e77684bf888ad6d1b4844d220196a

Modified Files
--------------
meson_options.txt                        |   3 +
src/test/fuzzing/fuzz_b64decode.c        |  98 ++++++++++++++
src/test/fuzzing/fuzz_conninfo.c         | 105 +++++++++++++++
src/test/fuzzing/fuzz_json.c             | 104 +++++++++++++++
src/test/fuzzing/fuzz_json_incremental.c | 127 ++++++++++++++++++
src/test/fuzzing/fuzz_parsepgarray.c     | 102 +++++++++++++++
src/test/fuzzing/fuzz_pgbench_expr.c     | 211 ++++++++++++++++++++++++++++++
src/test/fuzzing/fuzz_pglz.c             | 127 ++++++++++++++++++
src/test/fuzzing/fuzz_rawparser.c        | 162 +++++++++++++++++++++++
src/test/fuzzing/fuzz_regex.c            | 193 +++++++++++++++++++++++++++
src/test/fuzzing/fuzz_saslprep.c         | 104 +++++++++++++++
src/test/fuzzing/fuzz_typeinput.c        | 218 +++++++++++++++++++++++++++++++
src/test/fuzzing/fuzz_unescapebytea.c    | 103 +++++++++++++++
src/test/fuzzing/meson.build             | 203 ++++++++++++++++++++++++++++
src/test/meson.build                     |   2 +
15 files changed, 1862 insertions(+)