Add feature support for Panorama

[fpluch]

Description

Added :

• Device / General Settings
• Device / High Availability
• Device / High Availability / Link Monitoring
• Device / High Availability / Path Monitoring

Motivation and Context

Some features like the ones mentioned above are missing from Panorama connector. Those would need to be added in the terraform provider

How Has This Been Tested?

Added tests from go test for all 4 features, Tested on a development Panorama also.

Screenshots (if appropriate)

Types of changes

• New feature (non-breaking change which adds functionality)

Note: Hostname attribute in the General Settings has been moved to "omitempty" in order to be able to have generic templates that do not override any more specific template or any locally configured value.

Checklist

• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes if appropriate.
• All new and existing tests passed.