feat(kafka-logger): add TLS support for Kafka brokers

[ecsimsw]

Description

• Add TLS option to kafka-logger and error-log-logger plugins for connecting to TLS-enabled Kafka brokers such as AWS MSK.
• APISIX uses lua-resty-kafka for Kafka produce, and lua-resty-kafka already supports ssl/ssl_verify in broker_config.
• However, APISIX plugins were not exposing these options to users.
• This PR adds a tls object schema so users can enable TLS connections.
• The tls object design allows future extension for mTLS support.

Changes

• kafka-logger.lua : add tls schema + pass ssl/ssl_verify to broker_config
• error-log-logger.lua : same change
• kafka-logger-tls.t : schema validation + integration tests via SSL port (9093)
• error-log-logger-kafka.t : schema validation + integration tests for TLS
• kafka-logger.md, error-log-logger.md : add tls attributes and usage examples

Which issue(s) this PR fixes:

• Fixes kafka security_protocol Does kafka support SASL_SSL? #9353

Verification

• Tested against AWS MSK (Real world, TLS-only) : TLS handshake and metadata fetch confirmed working.
• luacheck: passed locally (0 warnings / 0 errors)
• check-test-code-style.sh: passed locally
• lj-releng: requires LuaJIT 2.1 (not available locally), to be verified by CI

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)