Add literal style support for OpenApiYamlWriter

[foriequal0]

OpenAPI allows description to be written in Markdown. Markdown descriptions can be multiple lines.
Current OpenApiYamlWriter escapes all control characters and joins and serializes them in one line.

I read description from external README.md file, and put it to document.Info.Description, and publishes the generated OpenAPI document.
Users can diff with previous versions of documents to track any public changes.
However, it is quite daunting when you have to look these long concatenated Markdown description to find diffs.

With literal style block scalar, I can preserve my line breaks of README.md as is.

[msftclas]

All CLA requirements met.

[darrelmiller]

@foriequal0 Thank you for this submission. This is a great addition. I am going to try it out and get it released ASAP.

[darrelmiller]

There is just one issue that concerns me here. Using this input:

openapi: 3.0.0
info:
  title: asdads 
  version: 1.0.0
paths:
  /: 
    get:
      description: |
                     Hello
                     World
      responses:
        200:
          description: ok

the output is

openapi: 3.0.1
info:
  title: asdads
  version: 1.0.0
paths:
  /:
    get:
      description: |+
        Hello
        World

      responses:
        '200':
          description: ok

If this output is used as input then the "keep" chomping indicator will cause an extra newline to be introduced. Would it not be better to use the "clip" chomping indicator as the default output?

[foriequal0]

I also found that "clip" indicator introduces a trailling newline even if the original input doesn't have one.
I'll push more commits to handle chomping indicator correctly.

[foriequal0]

Also I realized that I don't have to preserve linebreaks. The spec clearly says:

Line breaks inside scalar content must be normalized by the YAML processor. Each such line break must be parsed into a single line feed character. The original line break format is a presentation detail and must not be used to convey content information.
https://yaml.org/spec/1.2/spec.html#id2774608

I'll remove clunky line break handlings too.