ci: refactor specs with useful boundary from LS-core extension

[yaauie]

The existing specs reach into the internals of the Geoip Database Manager that may be provided by LS core, but those details are changing with Logstash 8.12's introduction of a Geoip Database Manager as a core feature.

Here we make a number of changes to port the existing specs to a form that validates the same desired behaviour without needing to reach into the implementation of the LS-core-provided extension.

1. we can trust that if LogStash::Filters::GeoIP::DatabaseManager is available:
   • it will be a singleton that responds to subscribe_database_path and unsubscribe_database_path, and behaves as-specified in Logstash core, so we can write our spec in terms of those two methods only
   • it and its Extension will be loaded before an instance of this plugin can be instantiated, and therefore we should side-load the extension directly before running tests
2. we extend the specs to cover the OSS-only use-case, and expand the test matrix to cover OSS releases (depends on https://github.com/logstash-plugins/.ci/pull/56)

NOTE: I've captured the output of the specs as-run on LS 7.x, 8.x SNAPSHOT=false (old manager), 8.x SNAPSHOT=true (after elastic/logstash#15348), etc:

https://gist.github.com/yaauie/fa042e9e22d08ad2a2c9f279f41f505e

Things to look for:

• when DISTRIBUTION=oss, x-pack isn't loaded and specs show behaviour without database manager

  ci-logstash-1  | X-PACK is not available

  ci-logstash-1  |     Logstash >= 7.14
  ci-logstash-1  |       OSS-only
  ci-logstash-1  |         load_database_manager? should be false
  ci-logstash-1  |         select_database_path without path setting
  ci-logstash-1  |           should be default

  ci-logstash-1  |   when no database_path is given
  ci-logstash-1  |     when geoip database management is not available
  ci-logstash-1  |       enriches events
  ci-logstash-1  |       uses a plugin-vendored database

• when 7.x or 8.x SNAPSHOT=false, the old extension is loaded

  ci-logstash-1  | ADDING LOGSTASH X-PACK to load path: /usr/share/logstash/x-pack/lib
  ci-logstash-1  | loading logstash extension for geoip: LogStash::Filters::Geoip::Extension

• when 8.x SNAPSHOT=true the new extension is loaded

  ci-logstash-1  | ADDING LOGSTASH X-PACK to load path: /usr/share/logstash/x-pack/lib
  ci-logstash-1  | loading logstash extension for geoip: LogStash::GeoipDatabaseManagement::Extension

• when default distro and dbm is available, we validate the behaviour:

  ci-logstash-1  |     Logstash >= 7.14
  ci-logstash-1  |       default distro
  ci-logstash-1  |         load_database_manager? should be true

  ci-logstash-1  |   when no database_path is given
  ci-logstash-1  |     when geoip database management is available
  ci-logstash-1  |       uses a managed database
  ci-logstash-1  |       enriches events