[core] Normalizing vendor extension naming (#5192)

* [core] Normalizing vendor extension naming

According to [OAS 2.0][1] and [OAS 3.0][2] Specifications:

> Allows extensions to the OpenAPI Schema. The field name MUST begin with x-,
>  for example, x-internal-id. The value can be null, a primitive, an array or an object.
>  Can have any valid JSON format value.

This commit attempts to define a [clear identifier design format][3] of
maintaining lower-kebab casing and following the x- prefix defined by
OAI Specification.

Following a convention that matches that used by others (see [autorest][4]), we will remove
any confusion about naming strategies for template authors and
customizers. Following the lower-kebab convention will allow us to
convert from camelCase and missing prefixes to the desired format. For
example, these conversions are simple to make for template consistency:

* customValue => x-custom-value
* x-customValue => x-custom-value
* x-custom-value => x-custom-value

This convention also allows us to define a single standard for use
across all generators. This means no occurrence of x-operationId in one
generator and x-operation-id in another.

[1]: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#patterned-objects
[2]: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.2.md#specificationExtensions
[3]: https://tools.ietf.org/html/draft-wilde-registries-01#section-3.4
[4]: https://github.com/Azure/autorest/tree/master/docs/extensions

* Incorporate feedback to avoid race/blocking in OnceLogger

* Remove unnecessary additional log config

* Add tests,comments for OnceLogger

* Test caffeine cache with FakeTicker

modules/openapi-generator-cli/src/main/resources/logback.xml

@@ -53,16 +53,6 @@
             <level>ERROR</level>
         </filter>
     </appender>
-    <appender name="ONCELOGGER_COLOR" class="ch.qos.logback.core.ConsoleAppender">
-        <target>System.err</target>
-        <withJansi>true</withJansi>
-        <evaluator class="ch.qos.logback.classic.boolex.OnMarkerEvaluator">
-            <marker>ONCE</marker>
-        </evaluator>
-        <encoder class="ch.qos.logback.classic.encoder.PatternLayoutEncoder">
-            <pattern>[%thread] %highlight(%-5level) %logger{36} - %red(%msg)%n</pattern>
-        </encoder>
-    </appender>
 
     <logger name="io.swagger" level="warn">
         <!-- Colorize by passing -Dcolor -->