support custom multi-form data fields in c8y api

[reubenmiller]

Allow users to control the form-data fields in the c8y api command (similar to what curl allows).

curl https://$C8Y_DOMAIN/example -H "$C8Y_HEADER" -F file=@./mydevice01.pem -F "parentAssetId=1111" -F "assetTypeId=2222"

[reubenmiller]

The api extensions commands would also benefit from being able to have more control over setting specific form data values via other means like bodyTemplates etc.