[Feature Request]: Allow skipping the 'Agent Install' tasks when self-hosted agents are in use and have limited Internet connectivity

### Description

## Problem space

Currently the CARML CI has a task in the pipeline templates that performs agent's configuration for pre-requisites (i.e. PowerShell Azure Modules), which requires the agent to have access to PS Gallery.


In isolated environments, it can be difficult to allow agents to have access to the PSGallery, and the pre-requisites can be fetched from a different source.

## Feature request

Allow skipping the tasks related to the agent setup as a switch in the `settings.yml`

Example:


- Create a switch called `installAgentPreRequisites` and set it to `true` as default.
- Create a condition on the tasks for agent install in all pipeline job templates to skip this stage `condition: eq(variables['installAgentPreRequisites'], true)`


## Another option

We could also use this switch to have a value like `validateOnly`, `installOnly`, `none` to support a scenario where we'd want to validate that the pre-requisites are installed opposed to installing them