Invoke additional steps in pipeline templates

In an effort to improve the pipeline jobs in the `source-build-reference-packages` (SBRP) repository, there is a requirement to integrate a testing step into the `source-build.yml` steps template. However, modifying the arcade templates within the SBRP repository is not possible because these modifications will be overridden by arcade dependency flows.

Context (from [Pull Request #824](https://github.com/dotnet/source-build-reference-packages/pull/824#discussion_r1391414150)):

> Maybe it would make sense to provide a mechanism to invoke additional steps that are passed in. It might not make sense to expose that throughout all of the templates but more so if you just wanted to reuse the steps template directly.

Another alternative would be to do something similar to https://github.com/dotnet/arcade/blob/69e73774ddfdacb1eea03e86d0c6ffcdb94eb4dd/eng/common/templates/job/job.yml#L32