[PROPOSAL] Replace direct plugin download by maven dependencies for bcw

[dblock]

What/Why

What are you proposing?

Coming from opensearch-project/cross-cluster-replication#566 (comment), replace any direct download of the latest plugin zip such as opensearch-project/job-scheduler#242 by a maven dependency.

Document how to do this in https://github.com/opensearch-project/opensearch-plugins/blob/main/TESTING.md#backwards-compatibility-testing

What users have asked for this feature?

See opensearch-project/cross-cluster-replication#566 (comment). This would reduce the amount of code needed for bcw.

What problems are you trying to solve?

Standardize dependency management for bcw across plugins.

[rursprung]

before this is done in the other repositories, opensearch-project/OpenSearch#4589 should be solved so that the code can be further simplified. the current hack with RegularFile creates a lot of unnecessary overhead and goes directly against what the javadoc of RegularFile says (i.e.: never construct it yourself).