Provide an interface for fuzzing sequence producer plugins

[embg]

Fuzz-testing for the external sequence producer API was introduced in PR #3437. However, the setup in #3437 only allows fuzzers to exercise the implementation of the API on the libzstd side (the code in the core zstd library which interacts with a plugin). This PR allows us to fuzz the plugin code itself, which is a requirement for us to use plugins (such as hardware accelerators) in production.

Here is an example showing how this new interface allows us to link plugin code into the fuzzer build:

$ make corpora
$ make -C seq_prod_fuzz_example/
$ python3 ./fuzz.py build all --custom-seq-prod=seq_prod_fuzz_example/example_seq_prod.o --enable-fuzzer --enable-asan --enable-ubsan --cc clang --cxx clang++
$ python3 ./fuzz.py libfuzzer simple_round_trip
$ python3 ./fuzz.py libfuzzer stream_round_trip
$ python3 ./fuzz.py libfuzzer dictionary_round_trip
$ # etc...

[terrelln]

Looks good. Can you also add the documentation about how to build & run the fuzzer in seq_prod_fuzz_example/README.md?

[GarenJian-Intel]

Hi @embg, now only simple_round_trip is enabled, will other fuzzers be enabled in this PR or other new PRs? If other fuzzers are ready, could you @ me so that I can try them ASAP?

[embg]

@GarenJian-Intel Added support for the following fuzzers:

block_round_trip
decompress_dstSize_tooSmall
dictionary_decompress
dictionary_loader
dictionary_round_trip
dictionary_stream_round_trip
raw_dictionary_round_trip
sequence_compression_api
simple_compress
simple_round_trip
stream_round_trip

Also, responded to all nits from @terrelln. Merging after CI passes.