Ensure type-level Javadoc in every class, comply with guidelines for
80 char on Javadoc, and minor polish.

The `WebClient` has a new exception hierarchy:

* `WebClientException` is the root of all exceptions thrown by the
`WebClient`
* `WebClientResponseException` are all exceptions associated with
specific HTTP response status codes
* `WebClientErrorException` and `WebServerErrorException` are
respectively for 4xx and 5xx HTTP status codes

`ResponseExtractor` implementations are adapted to optionally throw
exceptions if it's impossible to extract the relevant parts of the
response (e.g. extracting the response body if the response is a 404).

This commit also introduces `ResponseErrorHandler`s that take care of
the whole exception mapping infrastructure. Since
`WebClientResponseException`s provide the status, headers and response
body, we also need a dedicated mechanism to extract information from the
response body at that level.

The `BodyExtractors` are responsible for extracting that information
from the exception, given they are provided with all the information
they need; in that case, message decoders are required.

To convey all this new information downstream, the `WebClient` now wraps
the message converters and response error handler instances into a
dedicated `WebClientConfig` object.

Issue: SPR-14467

Closes gh-1108

 - Added Woodstox before Aalto in the spring-web build, so that Woodstox
   is used as StAX implementation, and not the less featured Aalto.
 - Hardcoded Aalto dependency in XmlEventDecoder, instead of relying on
   the StAX XMLInputFactory.

This commit also aligns ResourceUrlProvider's and RequestMappingInfo's UrlPathHelper setter/getter signatures.

Issue: SPR-14454

Issue: SPR-13188

Issue: SPR-14432

Issue: SPR-13188

Issue: SPR-14430

This commit also removes outdated support classes for Oracle, GlassFish, JBoss.

Issue: SPR-14429

This commit also fixes broken javadoc links and code references.

Issue: SPR-13188

This commit also removes the corresponding deprecated Servlet MVC variant and updates DispatcherServlet.properties to point to RequestMappingHandlerMapping/Adapter by default.

Issue: SPR-14129

Issue: SPR-13189

Issue: SPR-14344

Issue: SPR-14422

Closes gh-1097

Issue: SPR-14422

Issue: SPR-14420

Issue: SPR-14412

This commit adds a `ClientHttpRequestInterceptor` that applies a BASIC
authorization header for each request.

It can be used as follows:

```
BasicAuthorizationInterceptor basicAuthorization =
    new BasicAuthorizationInterceptor("user", "secret");
restTemplate.getInterceptors().add(basicAuthorization);
```

Issue: SPR-14412

Issue: SPR-14405

As of 4.3 ResourceHttpRequestHandler delegates to the configured
ContentNegotiationManager, or one created internally, to look up
the media type for are resource.

This commit ensures the internally created ContentNegotiationManager is
correctly injected with the ServletContext through which it can perform
lookups as before.

Also the ServletPathContentNegotiationStrategy now checks the
ServletContext first and then delegates to its parent the
PathContentNegotiationStrategy and not vice versa. This is
consistent with how handleNoMatch (also in the same class) works
and also matches how ResourceHttpRequestHandler worked before 4.3.

Issue: SPR-14368

Issue: SPR-14406

Issue: SPR-14372

Issue: SPR-14358

Issue: SPR-14347

Issue: SPR-14338

Issue: SPR-14291