Move Application CRD client from application-operator folder to central-application-gateway folder

[akgalwas]

Description

Move contents of components/application-operator/pkg to components/central-application-gateway/pkg and remove components/application-operator folder.

Tasks needed for 1.21

• Remove application operator #16660
• Migrate Compass Runtime Agent after Application Operator removal #16677
• Migrate connectivity validator after operator removal #16693
• Clean up go mod replace  #16712

Tasks follow-ups

• Migrate application connector tests #16692

Reasons

Application Operator component has been removed some time ago, so the folder components/application-operator/pkg contains Application CRD client only. By removing components/application-operator folder we reduce maintenance effort (no need to update go.mod) and make step towards Application Connector module.