[META]: support opaque `api_key` values for stack components

[yaauie]

Many plugins that communicate with Elasticsearch accept an api_key parameter, which they expect to be in the name:key format. This value is base64-encoded into an opaque value before being used in an Authorization: ApiKey #{opaque} HTTP header.

To reduce friction, we should accept the opaque value for api_key anywhere that we accept the name:key format api_key. We can do so by assuming that the value is opaque unless it includes a colon (:) character.

plugin	name:key	opaque
logstash-output-elasticsearch	✅	❌
logstash-filter-elasticsearch	✅	❌
logstash-input-elasticsearch	✅	❌
logstash-filter-elastic_integration	✅	✅

• support opaque api_key in Elasticsearch Output
• support opaque api_key in Elasticsearch Filter
• support opaque api_key in Elasticsearch Input
• support opaque api_key in Elastic Integration Filter
• support opaque api_key in LS-Core Central Management
• support opaque api_key in LS-Core Stack Monitoring

Notes:

• Plugins in logstash-integration-elastic_enterprise_search have an api_key parameter, but the underlying client provides it verbatim as a Bearer token.
• LS Core's Central Management and Stack Monitoring both borrow connections from a stand-alone Elasticsearch Output plugin, to which they provide a configured api_key without modification; they should work for free once the Elasticsearch Output has been updated.