Added capability to put login banner using general_settings

[jinfluenza]

Description

This PR is to add capability to put login banner using general settings resource block

Motivation and Context

As we try to automate further, I feel that a lot of consumer will benefit from having an ability to add login banner programmatically. While making this change I noticed that Pango already has login banner in its config, so I just needed to enable login banner within provider code base.

How Has This Been Tested?

This was tested via Panos environment

• Before the change

• Adding the login banner in the resource block

• Able to read the changes that are being made

• After applying and running the firewall-commit binary

This was tested on VM-series of Pan OS deployed in Cloud and applying the changes using the Terraform to the instance.

Types of changes

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

Checklist

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

[welcome-to-palo-alto-networks[bot]]

:tada: Thanks for opening this pull request! We really appreciate contributors like you! :raised_hands:

[jinfluenza]

Hi @shinmog! Just wanted to see if you had a chance to look at this PR. I have more coming to add more features, but wanted to see I am following your structure with this small PR. lmk if you have any suggestions!

[shinmog]

@jinfluenza Rebase when you get a chance and I can merge this enhancement

[jinfluenza]

@shinmog it has been rebased. thank you!

[AYDEV-FR]

I'm also interested in this merge request. Can it be merged?