WIP on-prem agent Azure pipeline pool logic

[MarkZither]

This PR addresses #1019, I added a new AzurePipelineImage of OnPrem and a string array property OnPremPools.

The code in AzurePipelinesAttribute is a bit of copy and paste of GetStage, GetJob and GetSteps to allow the pool name to be passed as a string rather than passing the AzurePipelineImage and the nested ternary in AzurePipelinesCacheStep AdjustedPath needs improving, but it is currently working as I hoped.

Input like

[AzurePipelines(
    suffix: null,
    AzurePipelinesImage.OnPrem,
    OnPremPools = new[] { "my-onprem-pool" },

give me the output I want

stages:
  - stage: my_onprem_pool
    displayName: 'my-onprem-pool'
    dependsOn: [  ]
    pool: 'my-onprem-pool'

I confirm that the pull-request:

• [ x ] Follows the contribution guidelines
• [ x ] Is based on my own work
• [ x ] Is in compliance with my employer

[matkoch]

I don't think that's a great implementation. OnPrem is not an image, so why put it in AzurePipelinesImage ?