[BUG][MARKDOWN] Parameter and property names are needlessly changed according to default specialCharReplacements mapping

Created by: TanyaS

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

Different languages have different set of acceptable symbols for identifiers.
However, MarkdownDocumentationCodegen doesn't override specialCharReplacements mapping, DefaultCodegen#toParamName and DefaultCodegen#toVarName methods. This leads to parameter and variable names being processed according to default logic, for example:

  "components": {
    "schemas": {
      "Pet": {
        "type": "object",
        "properties": {
          "_name_ext_": {
            "type": "string"
          }

becomes

Name	Type	Description	Notes
UnderscorenameUnderscoreextUnderscore	String		[optional] [default to null]

openapi-generator version

4.3.1-SNAPSHOT

OpenAPI declaration file content or url

https://gist.github.com/TanyaS/d7067e018b42355695574d7f0e90512b

Command line used for generation

generate \
    -g markdown \
    -o markdown \
    -i openapi-spec-with-wierd-names.json \
    --language-specific-primitives string

Steps to reproduce

Generate markdown docs using provided spec and cli command.

Related issues/PRs

Suggest a fix

specialCharReplacements mapping can be overridden like so:

    @Override
    protected void initalizeSpecialCharacterMapping() {
        // escape only those symbols that can mess up markdown
        specialCharReplacements.put("\\", "\\\\");
        specialCharReplacements.put("/", "\\/");
        specialCharReplacements.put("`", "\\`");
        specialCharReplacements.put("*", "\\*");
        specialCharReplacements.put("_", "\\_");
        specialCharReplacements.put("[", "\\[");
        specialCharReplacements.put("]", "\\]");

        // todo Current markdown api and model mustache templates display properties and parameters in tables. It can be
        // commonly escaped with a backslash (e.g. GFM supports this). However, in some cases it may be necessary to
        // choose a different approach.
        specialCharReplacements.put("|", "\\|");
    }

And toParamName method can be overridden to omit camelization:

/**
     * Works identically to {@link DefaultCodegen#toParamName(String)} but doesn't camelize.
     *
     * @param name Codegen property object
     * @return the sanitized parameter name
     */
    @Override
    public String toParamName(String name) {
        if (reservedWords.contains(name)) {
            return escapeReservedWord(name);
        } else if (((CharSequence) name).chars().anyMatch(character -> specialCharReplacements.keySet().contains("" + ((char) character)))) {
            return escape(name, specialCharReplacements, null, null);
        }
        return name;
    }