Support matrix values in step keys and concurrency groups

[apgrucza]

Is your feature request related to a problem? Please describe. A typical pipeline at my organisation deploys infrastructure to multiple AWS regions simultaneously with Terraform, via multiple similar plan and deploy steps. We use a unique concurrency_group for each region, and a unique key on each plan step. We want to use the build matrix feature as it would vastly simplify our pipeline definitions, but we're unable to due to the lack of support for matrix values in these attributes.

Describe the solution you'd like The ability to use matrix values (e.g. {{matrix}}) in the key and concurrency_group attributes.