feat: tag assembly task

[ingarabr]

Add a tag for the assembly tasks. This can be used to restrict resource usages.

Why do we need this?

Version 2 of sbt-assembly uses more resources. This is problematic when executing assembly on a multi-module project causing different types of errors. I've encountered memory and to many open files issue. Adding a tag gives the user of the plugin the option to restrict concurrent executions with Global / concurrentRestrictions += Tags.limit(Assembly.assemblyTag, 1)

Things to consider

• Should we reuse some of the existing tags? Sbt has defined both cpu, memory and disk tags.
• Should the plugin have a default limit on the tag?

[eed3si9n]

Should the plugin have a default limit on the tag?

I think it's a good idea to put default limit on 1.

[ingarabr]

I agree that a default limit of 1 can be a good default. Please take a look at a04849b