Support to set loadBalancerKey from request header

[alan-tang-tt]

When I write myself MyRule implements IRule, the key is always null, like below:

public class MyRule extends AbstractLoadBalancerRule {

    @Override
    public Server choose(Object key) {
          // here the key is always null
    }
}

By watching the source code, I find there's no where to set this loadBalancerKey. So I push this PR to resolve this problem by overriding FeignLoadBalancer#customizeLoadBalancerCommandBuilder method.

         // override FeignLoadBalancer#customizeLoadBalancerCommandBuilder
	@Override
	protected void customizeLoadBalancerCommandBuilder(RibbonRequest request, IClientConfig config, LoadBalancerCommand.Builder<RibbonResponse> builder) {
		builder.withServerLocator(request.request.headers().get("loadBalancerKey"));
	}

By this way, I can put my loadBalancerKey from feign.RequestInterceptor implementation like this:

public class MyRequestInterceptor implements RequestInterceptor {
    @Override
    public void apply(RequestTemplate template) {
        template.header("loadBalancerKey", "gray");
    }
}

Then in MyRule I can get the key:

public class MyRule extends AbstractLoadBalancerRule {

    @Override
    public Server choose(Object key) {
          // no the key equals to gray
    }
}

This is all.

[codecov]

Codecov Report

Merging #451 (fdbc0c7) into 2.2.x (e32b10b) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##              2.2.x     #451      +/-   ##
============================================
+ Coverage     77.80%   77.98%   +0.18%     
- Complexity      476      477       +1     
============================================
  Files            64       64              
  Lines          1883     1885       +2     
  Branches        265      265              
============================================
+ Hits           1465     1470       +5     
+ Misses          298      295       -3     
  Partials        120      120              

Impacted Files	Coverage Δ	Complexity Δ
...work/cloud/openfeign/ribbon/FeignLoadBalancer.java	70.14% <100.00%> (+0.91%)	7.00 <1.00> (+1.00)
...ncer/RetryableFeignBlockingLoadBalancerClient.java	70.76% <0.00%> (+4.61%)	11.00% <0.00%> (ø%)

[OlgaMaciaszek]

@alan-tang-tt Thanks for the PR. The idea is good, however some more work has to be done on the PR.
Please verify if the headers contain the header you want to use.
Please change the default header name to X-LodBalancer-Key.
Please allow changing header name via properties.
Please add tests.
Please add documentation.

[kimmking]

And consider other parameter like url parameter？