[Server][Spring] support interface and implementation classes for API controllers

[kapilguptavz]

added enhancement for java spring asked in OpenAPITools#426 (comment)

Reproduction Steps for java spring:

1. Create sample.yaml with endpoint containing only get method and generate server stub with below command:

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -i sample.yaml -g spring -o springserver

 sample.yaml(paths details with get)
	 paths:
	  /pets:
		get:
		  summary: List all pets
		  operationId: listPets
		  tags:
			- pets
		  parameters:
			- name: limit.....

2. Update sample.yaml with post method in exiting endpoint and generate server stub with same command given in step 1.
   updated sample.yaml (paths details with post method added)

		  paths:
			/pets:
			get:
			  summary: List all pets
			  operationId: listPets
			  tags:
				- pets
			  parameters:
				- name: limit
			post:
			  summary: Create a pet
			  operationId: createPets
			  tags:
				- pets
			  responses:
				'201':....

Result: In step 1, it will create PetsApi.java(interface ), PetsApiController.java(implementation class) as new and in 2nd step
it overwrites both file.

Enhancement with result:
added checks for file creation for java spring server for controller class.
Unit Test Case: Tested for multiple endpoints
sample.yaml

    paths:
	  /pets:
		get:
		  operationId: listPets
		  tags:
			- pets
		  parameters:
			- name: limit
			  in: query
			  description: How many items to return at one time (max 100)
			  required: false
			  schema:
				type: integer
				maximum: 100
				format: int32
		  responses:
			'200':
			 ....
		post:
			summary: Create a pet
			operationId: createPets
			tags:
			  - pets
			responses:
			  '201':
				description: Null response
			  default:
				description: unexpected error
				content:
				  application/json:
					schema:
					  $ref: "#/components/schemas/Error"
	  /pots/{potId}:
		get:
		  summary: Info for a specific pet
		  operationId: showPetById
		  tags:
			- pets
		  parameters:
			- name: potId
			  in: path
			  required: true
			  description: The id of the pet to retrieve
			  schema:
				type: string
		  responses:
			'200':`

Result: generated server stubs multiple times. It only overwrites interfaces (pets.api and pots.api). Implementation classes(PetsApiController.java and PotsApiController.java) are created once.
@cachescrubber @welshm @MelleD @atextor @manedev79 @javisst @borsch @banlevente @Zomzog @martin-mfg

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[saibalroyvz]

Code looks good..

[martin-mfg]

Hi @kapilguptavz, this PR is targetting your fork of OpenAPI Generator. But you probably wanted to target the original OpenAPI Generator repository.
Please take care of the one comment below and then open a new PR in the original OpenAPI Generator repo.
Also, thanks for your contribution!