Add the ability to change request uri dynamically in filters

[making]

This PR adds AbstractChangeRequestUriGatewayFilterFactory that enables the gateway to change URL dynamically using ServerWebExchange.
As a implementation, RequestHeaderToRequestUriGatewayFilterFactory comes with this PR that determine URL by a request header.
This factory will be pretty useful when implementing Cloud Foundry Route Service.

also fixes #276

https://twitter.com/Fitzoh/status/985423505947754496

[codecov-io]

Codecov Report

Merging #286 into master will increase coverage by 0.44%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #286      +/-   ##
==========================================
+ Coverage   75.88%   76.33%   +0.44%     
==========================================
  Files         105      107       +2     
  Lines        2613     2645      +32     
  Branches      191      191              
==========================================
+ Hits         1983     2019      +36     
+ Misses        524      520       -4     
  Partials      106      106

Impacted Files	Coverage Δ
...cloud/gateway/route/builder/GatewayFilterSpec.java	58.97% <100%> (+4.04%)	⬆️
...cloud/gateway/config/GatewayAutoConfiguration.java	97.95% <100%> (+0.02%)	⬆️
...RequestHeaderToRequestUriGatewayFilterFactory.java	100% <100%> (ø)
.../AbstractChangeRequestUriGatewayFilterFactory.java	100% <100%> (ø)
...ork/cloud/gateway/route/builder/PredicateSpec.java	53.65% <0%> (+9.75%)	⬆️

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[cah-andrew-fitzgerald]

Awesome!

The one thing I might consider would be pulling Function<ServerWebExchange, Optional<URI>> into a functional interface, but that could go either way

[Qsimple]

Hi, dear dev team:
how do you think about this code snapshot in RequestHeaderToRequestUriGatewayFilterFactory:

	@Override
	protected Optional<URI> determineRequestUri(ServerWebExchange exchange,
			NameConfig config) {
		String requestUrl = exchange.getRequest().getHeaders().getFirst(config.getName());
		return Optional.ofNullable(requestUrl).map(url -> {
			try {
				return new URL(url).toURI();
			}
			catch (MalformedURLException | URISyntaxException e) {
				log.info("Request url is invalid : url={}, error={}", requestUrl,
						e.getMessage());
				return null;
			}
		});
	}

if we need to change original uri to "lb://XXX", MalformedURLException will be catched and return null. I think this an common scenario.

                                                                                                         Best regards
                                                                                                          simple

[spencergibb]

Create a URI object directly, not a url first.

[Qsimple]

So, may I push a PR to fix it? I think some testcase needed to be modified to allowd "lb://XXX"

[spencergibb]

PRs welcome

[shiddarthbista]

@spencergibb
Is there a way to set the route uri dynamically based on a value in the request body during runtime?