[NET-9450] Add byPort function for grouping services by port number

[nathancoleman]

This PR adds a byPort function to consul-template.

Similar in nature to the existing byTag function, byPort will group services in a list by port number.

Example

Input

[
  {ID: "serviceA", Port: 8080},
  {ID: "serviceB", Port: 8080},
  {ID: "serviceC", Port: 9090}
]

should return

{
  8080: [
    {ID: "serviceA", Port: 8080},
    {ID: "serviceB", Port: 8080},
  ],
  9090: [
    {ID: "serviceC", Port: 9090}
  ]
}

HealthService, CatalogService and NomadService types are supported. Notably, CatalogSnippet (code) and NomadServicesSnippet (code) types are not supported since the responses from the corresponding API endpoints do not include port numbers for each service.

[nathancoleman]

I will dig into the CI failures tomorrow. I believe they're widespread issues not specific to this PR since the added tests pass locally.

[blake]

UX looks good to me.

[jm96441n]

LGTM!