feat: metal gateway get -i <id>

[displague]

Adds metal gateway get -i:

$ go run ./cmd/metal/ gateways get  -i $METAL_GATEWAY_ID                   
+--------------------------------------+-------+-------+------------------+--------+-------------------------------+-------------------+
|                  ID                  | METRO | VXLAN |    ADDRESSES     | STATE  |            CREATED            |        VRF        |
+--------------------------------------+-------+-------+------------------+--------+-------------------------------+-------------------+
| bd3162f4-4162-408b-8670-f70782fe763e | am    | 1002  | 192.168.100.2/22 | active | 2024-08-02 18:16:23 +0000 UTC | nutanix-vrf-bRkYc |
+--------------------------------------+-------+-------+------------------+--------+-------------------------------+-------------------+

$ go run ./cmd/metal/ gateways get  -p $METAL_PROJECT_ID 
+--------------------------------------+-------+-------+------------------+--------+-------------------------------+-------------------+
|                  ID                  | METRO | VXLAN |    ADDRESSES     | STATE  |            CREATED            |        VRF        |
+--------------------------------------+-------+-------+------------------+--------+-------------------------------+-------------------+
| bd3162f4-4162-408b-8670-f70782fe763e | am    | 1002  | 192.168.100.2/22 | active | 2024-08-02 18:16:23 +0000 UTC | nutanix-vrf-bRkYc |
+--------------------------------------+-------+-------+------------------+--------+-------------------------------+-------------------+

After unsettling project id in ~/.config/equinix/metal.yaml:

$ go run ./cmd/metal/ gateways get 
Error: either id or project-id should be set
Usage:
  metal gateway get [-p <project_UUID>] [-i <gateway_UUID>] [flags]

Aliases:
  get, list

Examples:

  # Lists Metal Gateways for project 3b0795ba-ec9a-4a9e-83a7-043e7e11407c:
  metal gateways get -p 3b0795ba-ec9a-4a9e-83a7-043e7e11407c
  
  # Gets a Metal Gateway with ID 8404b73c-d18f-4190-8c49-20bb17501f88:
  metal gateways get -i 8404b73c-d18f-4190-8c49-20bb17501f88

Flags:
  -h, --help                help for get
  -i, --id string           The UUID of a gateway.
  -p, --project-id string   The project's UUID. This flag is required, unless specified in the config created by metal init or set as METAL_PROJECT_ID environment variable.

Global Flags:
...

[displague]

Noting the relevance to equinix/equinix-sdk-go#64