Optimize plugins installation during packages' assembly

[f-galland]

Description:

Running the scripts/assemble.sh script locally results in packages being re-downloaded every time.

Downloading directly from the Maven repository is possible and can potentially speed up build times by caching downloads (whilst also automatically checking for integrity).

References:

• https://opensearch.org/blog/opensearch-plugin-zips-now-in-maven-repo/
• https://mvnrepository.com/artifact/org.opensearch.plugin/