add standard request Content-Type Header Route Predicator

Is your feature request related to a problem? Please describe.
Add support for request content-type Header Route Predicate Factory based on RFC requirements.
That will simplify filtering requests when clients use different software/environments especially for embedded systems.

Content-type is a standart header of HTTP request. Thus it has followign differences:

1. it's value is partly case-insensitive, partly could be sensitive.

The type, subtype, and parameter name tokens are case-insensitive.
Parameter values might or might not be case-sensitive, depending on
the semantics of the parameter name. ....A parameter value that matches the token production can be
transmitted either as a token or within a quoted-string. The quoted
and unquoted values are equivalent. For example, the following
examples are all equivalent, but the first is preferred for
consistency:

 text/html;charset=utf-8
 text/html;charset=UTF-8
 Text/HTML;Charset="utf-8"
 text/html; charset="utf-8"

2. Unlike some similar constructs in other header fields, media type parameters do not allow whitespace (even "bad" whitespace) around the "=" character.
3. Internet media types ought to be registered by IANA.

Describe the solution you'd like
something like:

spring:
  cloud:
    gateway:
      routes:
      - id: content_type_header_route
        uri: https://example.org
        predicates:
          content-type:
            media-type:
              type: text
              subtype: html
            parameter: 
             value: utf-8
             name: charset
             is-value-case-insensitive: yes #(default value)

That will require changes in org.springframework.cloud.gateway.handler.predicate package.
Describe alternatives you've considered
It's possible to use Header Route Predicate Factory with custom regexes, thus one needs to take care of case sensitivity, "=" sign spaces which make it less readable

Additional context
https://tools.ietf.org/html/rfc7231#section-3.1.1.1
https://www.iana.org/assignments/media-types/media-types.xhtml
https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/POST

(*) when applicable

Start with the existing Header Route Predicate Factory and the org.springframework.cloud.gateway.handler.predicate package named in the issue. Read RFC 7231 section 3.1.1.1 and compare the requested media-type and parameter matching rules with the proposed route configuration. Done means a standard Content-Type route predicate supports the stated case, quoting, whitespace, and parameter behavior.