Add support for Webtop management

[blagoySimandov]

Add support for Webtop management

This pull request implements the feature request outlined in issue #107, adding support for Webtop Creation, Deletion, Modification, and Retrieval to the F5 BIG-IP Go SDK.

Changes Implemented

1. Added new types and constants in apm.go:
   • WebtopType, CustomizationType, InitialState, and LinkType
   • BooledString for handling boolean values represented as strings in JSON payloads
2. Implemented Webtop, WebtopConfig, and WebtopRead structs in apm.go.
3. Added the following methods to the BigIP struct in apm.go:
   • CreateWebtop(ctx context.Context, webtop Webtop) error
   • DeleteWebtop(ctx context.Context, name string) error
   • GetWebtop(ctx context.Context, name string) (*WebtopRead, error)
   • ModifyWebtop(ctx context.Context, name string, webtop WebtopConfig) error
4. Created a new example in cmd/webtop/example.go to demonstrate the usage of the new Webtop functionality.

Implementation Details

• The Webtop struct represents the configuration for creating or updating a Webtop.
• The WebtopRead struct includes additional fields returned when retrieving a Webtop.
• The BooledString type handles the conversion between boolean values and their string representations in the API.

Testing

• Manual testing has been performed with the new example in cmd/webtop/example.go.

Additional Notes

• The ModifyWebtop method only allows updating the WebtopConfig fields, as per the API specifications.

Related Issue

Closes #107

Please review and let me know if any further changes or information are needed.