[cli] Enable recommendations on validate command

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.1.x, 4.0.x. Default: master.
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Description of the PR

see #142 (closed)

Adds a --recommend option to cli's validate.

validate with no switches has no functional change; it will print errors and exit with status code 1.

validate --recommends will work in the following way:

• If spec has errors + recommendations, output is written to stderr and exit code is 1
• If spec has no errors but recommendations, output is written to stdout, and exit code is 0

In all cases, no errors/recommendations outputs a message that there are no issues and status code is 0.

Sample spec:

openapi: "3.0.0"
servers:
  - url: http://petstore.swagger.io/v1
paths:
  /:
    get:
      responses:
        '200':
          description: A paged array of pets
          headers:
            x-next:
              description: A link to the next page of responses
              schema:
                type: string
components:
  schemas:
    Pet:
      required:
        - id
        - name
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
        tag:
          type: string
    Pets:
      type: array
      items:
        $ref: "#/components/schemas/Pet"
    Error:
      required:
        - code
        - message
      properties:
        code:
          type: integer
          format: int32
        message:
          type: string

Sample command(s):

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar validate --input-spec invalid.yaml --recommend

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar validate --input-spec invalid.yaml

Sample output:

Validating spec (.bak/invalid.yaml)
Errors:
	-attribute info is missing

[error] Spec has 1 errors.

Validating spec (.bak/invalid.yaml)
Errors:
	-attribute info is missing
Warnings:
	-Unused model: Pets
	-Unused model: Error
	-Unused model: Pet

[error] Spec has 1 errors.

note: output is almost identical to error/warnings from #251. There's possibly opportunity there once both are merged to combine outputs.