Leverage the AWS Batch service-linked role

[delagoya]

The current metaflow-computation module creates a custom service role.
AWS Batch introduced a service-linked role (SLR) a few years ago, which obviates this need.

The SLR is also a requirement for in-place compute environment updates for an expanded set of compute environment properties. SLRs are created automatically on the first compute environment creation.

There is still a use-case for user-defined service roles, but this is a legacy feature and should be used intentionally.
I propose to simplify the computation submodule to remove the created custom service role, thus defaulting to the SLR, and creating a new variable that allows a user to provide a custom service role that they created outside of the module.