Avoid Manticore and use Schema Registry client to check schemas

[andsel]

During the registration of the Kafka input, if schema registry is required, the plugins does some checks and uses the ReST API, using Manticore to issue the requests:
https://github.com/logstash-plugins/logstash-integration-kafka/blob/0462434fc6cf2c1de0d9314bbeb0c0840473d223/lib/logstash/plugin_mixins/kafka/avro_schema_registry.rb#LL71C16-L71C25

Avoid to use Manticore and leverage to the SchemaRegistryClient class to execute those checks.
This client is already bundled:

logstash-integration-kafka/build.gradle

Line 58 in 0462434

implementation("io.confluent:kafka-schema-registry-client:${confluentKafkaVersion}") {

[robbavey]

@andsel Is this still important/worthwhile?

[andsel]

In my opinion yes, using the client library provided by kafka-schema-registry-client in synch with the schema registry version could be safer. If we plan to support multiple versions (majors) of the schema registry, relaying on the HTTP API could permit to avoid version lock-in.