fastly healthcheck create is missing option to specify custom headers

[ccampbell]

Is your feature request related to a problem? Please describe.
The create healthcheck endpoint in the fastly API has a headers option where you can specify an array of headers to attach to a healthcheck.

See: https://www.fastly.com/documentation/reference/api/services/healthcheck/

Describe the solution you'd like
I would like to be able to set this field using the CLI

Describe alternatives you've considered
I have tried using API client libraries which also seem to have some trouble with this field.