Allow overriding of plugin metadata files in integration tests

[mark-vieira]

Provide a mechanism to override certain metadata in Elasticsearch plugins when installing them into test clusters. This first iteration provides methods for supplying overrides to the plugin-descriptor.properties and entitlement-policy.yaml files. When configuring a test cluster to install a plugin you can now optionally configure overrides by way of a function which receives the previous file's contents and returns a Resource to be used as the new contents.

An example of replacing an entitlement in the plugin policy file could look like so:

    @ClassRule
    public static ElasticsearchCluster cluster = ElasticsearchCluster.local()
        .plugin(
            "discovery-gce",
            spec -> spec.withEntitlementsOverride(
                original -> Resource.fromString(original.replace("set_https_connection_properties", "create_class_loader"))
            )
        )
        .build();

[elasticsearchmachine]

Pinging @elastic/es-delivery (Team:Delivery)

[rjernst]

LGTM

[elasticsearchmachine]

💚 Backport successful

Status	Branch	Result
✅	8.x
✅	8.17
✅	8.16