logstash: Support HTTP basic auth for logstash queries

Ahmon Dancyrequested to merge
master-I30f17105e21d7dd0e2b7bf1ff31db6620e2b3d54 into master

Enabling the OpenSearch security plugin requires clients to
authenticate. Add the optional logstash_credentials_file setting. Its
first line that is neither blank nor a #-prefixed comment supplies the
"username:password" pair. When set, logstash queries send an
Authorization header. When unset, queries are unauthenticated as before.

An unreadable or unparsable file raises ValueError when the Logstash
client is constructed.

Bug: T434114

Edited by Ahmon Dancy

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