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}") {