validate name field value

[fho]

According to the admin panel the name field is only allowed to contain "(letters and numbers only)" (https://panel.bunny.net/pullzones/add).

Add a validation function that ensures it.

[fho]

According to the API error message, it also supports hyphens:

Error: creating pull zone failed: http-request to https://api.bunny.net/pullzone failed: Bad Request (400), pullzone.validation: Name, PullZone name must consist of numbers, letters and hyphens only.