[vscode-spring-boot] Provide better Spring component code completion experience on class and method level

[nickzhums]

Method-level

• Users might need to deal with CRUD mapping such as @GetMapping, @PostMapping very often. Therefore, another enhancement is – when user starts typing @ symbol, we can pre-define a list of most often used CRUD mappings (@Get/@Put/@Delete/@post) and show them in the code completion list. Currently, the default suggestions do not contain all frequent Spring mappings.
  Class-level
• Besides method level completion enhancement, we could also consider adding better support for @bean / @component / @service when user is typing on the class definition. For example, when user creates an empty class in a Spring app, and starts to type “@”, we should make sure to provide the most frequently used Spring class annotations.

(cc: @martinlippert @Eskibear)