[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)

[martinlippert]

The first part of this issue is done, the code completion snippets for request mapping methods nicely show up when you start typing @ in the editor, in case you are inside a controller class.

I will extract the second part of this issue (the things on the class level) into a separate issue, also to discuss in more detail what exactly we would like to do there.