fix: Use pointers for integer fields in GTM, LTM, and Sys structs to support explicit zero values

[waltzbucks]

Summary

This PR addresses an issue where integer fields with a value of 0 were being omitted from JSON payloads sent to the BIG-IP API
due to the omitempty struct tag. This behavior caused problems in newer BIG-IP versions where default values for certain fields
(like GTM pool member ratio) have changed from 0 to 1. By changing these fields from int to *int, the SDK can now distinguish
between a missing value (nil) and an explicit zero (0), ensuring the intended configuration is applied.

Problem

In recent BIG-IP versions (e.g., 11.6.1 HF1+), the default value for GTM pool member ratio changed to 1. When a user explicitly
set ratio = 0:

1. The SDK saw 0 as the "Zero Value" for the int type.
2. The omitempty tag caused the field to be removed from the JSON request.
3. BIG-IP received the request without the ratio field and applied its internal default of 1.

This issue also affected other fields such as member_order, QoS settings, and LTM node/member ratios.

Changes

• gtm.go:
  • Updated GtmPoolMembers struct: Ratio and MemberOrder changed to *int.

Impact

• Users can now explicitly set 0 for these fields, and it will be correctly sent to the BIG-IP API.
• This change is backward compatible for most use cases but improves stability across different BIG-IP versions.
• Fixes issues where Terraform or other SDK consumers could not set certain values to zero.

Related Issue

(#130) Change GTM Pool Member Ratio to *int to support explicit 0 values