Upgrade security posture of grpc_json_transcoder

[nareddyt]

Commit Message:
Upgrade security posture of grpc_json_transcoder

Additional Description:
Follow-up on #9334 to upgrade the security posture of the filter to robust_to_untrusted_downstream.

Since the last discussion, we have:

• Added encoder and decoder buffer limits to the filter. grpc_json_transcoder: Adhere to decoder and encoder buffer limits #14906
• Improved test coverage to 95%.
• Improved fuzz coverage of the filter to 40% via the uber filter fuzzer.

Note the filter depends on the external library https://github.com/grpc-ecosystem/grpc-httpjson-transcoding. We followed the external dependency policy to add CI, add SECURITY.md, improve fuzz coverage, and update the library's dependencies.

Risk Level: N/A
Testing: None
Docs Changes: None
Release Notes: None
Platform Specific Features: None

Signed-off-by: Teju Nareddy [email protected]

[nareddyt]

cc @qiwzhang @yanavlasov