Change key config type to Password.

[mashhurs]

Description

This PR ensures to protect the :key from leaks in the debug logs.

• Closes Change key config to Password type. #70

Test

# config
Config:
      input {
        stdin {}
      }
      filter {
          fingerprint {
            key => "super-secret"
          }
      }
      output {
          stdout {
              codec => rubydebug
          }
      }

# Log before change
[2022-12-05T11:15:06,403][DEBUG][logstash.plugins.registry] On demand adding plugin to the registry {:name=>"fingerprint", :type=>"filter", :class=>LogStash::Filters::Fingerprint}
[2022-12-05T11:15:06,406][DEBUG][logstash.filters.fingerprint] config LogStash::Filters::Fingerprint/@key = "super-secret"

# Log after change
[2022-12-05T11:17:12,124][DEBUG][logstash.plugins.registry] On demand adding plugin to the registry {:name=>"fingerprint", :type=>"filter", :class=>LogStash::Filters::Fingerprint}
[2022-12-05T11:17:12,128][DEBUG][logstash.filters.fingerprint] config LogStash::Filters::Fingerprint/@key = <password>

# Unit tests
 mashhur:☆ rspec logstash-plugins/logstash-filter-fingerprint/spec/filters/fingerprint_spec.rb
    Sending Logstash logs to null which is now configured via log4j2.properties
    Run options: exclude {:integration=>true, :redis=>true, :socket=>true, :performance=>true, :couchdb=>true, :elasticsearch=>true, :elasticsearch_secure=>true, :export_cypher=>true, :windows=>true}

    Randomized with seed 63435
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    129 examples, 0 failures

    Randomized with seed 63435

[roaksoax]

lgtm!

[mashhurs]

Merging this commit and I believe the error is not related to current changes.
The fix for error already landed with this PR and I will investigate details.

Make sure that `gem install murmurhash3 -v '0.1.7' --source