add x-delimiter for style/explode customization

[karenetheridge]

I would like to do this:

components:
  headers:
    Surrogate-Key:
      description: see https://docs.fastly.com/en/guides/working-with-surrogate-keys
      style: simple  # default
      explode: false # default
      x-delimiter: ' '
      schema:
        type: array
        items:
          type: string
          pattern: ^myapp-

..because this header is formatted as space-separated strings, rather than comma-separated.

[karenetheridge]

I would suggest that the delimiter be expressed as a regular expression, rather than a string, for maximum flexibility.

This also naturally leads to us deriving the default values for various parameter types:

• for path, delimiter defaults to , (when style=simple)
• for header, delimiter defaults to \s*,\s* (where style=simple)
• for query, delimiter defaults to & (when style=form)
• for cookie, delimiter defaults to \s*;\s* (when style=form)

[karenetheridge]

note: do not trust https://swagger.io/docs/specification/serialization/#query - there are typos here!

[karenetheridge]

This is done in version 0.049.