When the user generates a platform, we need to know the platform ID it's
linked to in the holos server. If there is no platform with the same
name, the `holos generate platform` command should error out.
This is necessary because the first thing we want to show is pushing an
updated form to `holos server`. To update the web ui the CLI needs to
know the platform ID to update.
This patch modifies the generate command to obtain a list of platforms
for the org and verify the generated name matches one of the platforms
that already exists.
A future patch could have the `generate platform` command call the
`holos.platform.v1alpha1.PlatformService.CreatePlatform` method if the
platform isn't found.
Results:
```sh
holos generate platform bare
```
```txt
4:15PM INF generate.go:77 wrote platform.metadata.json version=0.77.1 platform_id=018f826d-85a8-751f-96d0-0d2bf70df909 path=/home/jeff/holos/platform.metadata.json
4:15PM INF generate.go:89 generated platform bare version=0.77.1 platform_id=018f826d-85a8-751f-96d0-0d2bf70df909 path=/home/jeff/holos
```
```sh
cat platform.metadata.json
```
```json
{
"id": "018f826d-85a8-751f-96d0-0d2bf70df909",
"name": "bare",
"display_name": "Bare Platform"
}
```
