feat(selector/global.go, selector/selector_test.go, transport/http/cl…

[chencheng8888]

…ient.go, transport/http/client_test.go): add an option to choose the load balancing algorithm for HTTP

• In selector/global.go, add function NewSelector to create a selector.Builder
• In selector/selector_test.go, add a test for NewSelector
• In transport/http/client.go, add 'selectBuilder' (selector.Builder) in clientOptions, and add function 'WithSelectBuilder' to set it. Also, adjust the function 'NewClient'
• In transport/http/client_test.go, add a test for WithSelectBuilder

Description (what this PR does / why we need it):

给http添加能够自主选择负载均衡算法，而不借助全局的selectBuilder。其实是因为在我想使用kratos的负载均衡的时候，看官方文档的示例，其实不太清楚具体如何去使用，也是看了源码才知道，kratos是通过全局变量来设置的，所以我就想像通过向http.NewClient添加选择项(option)来设置要使用哪个负载均衡算法，如

// new http client
	hConn, err := http.NewClient(
		context.Background(),
		http.WithMiddleware(
			recovery.Recovery(),
		),
		http.WithEndpoint("discovery:///helloworld"),
		http.WithDiscovery(r),
		http.WithNodeFilter(filter.Version("2.0.0")),
		selector.WithSelectorBuilder(wrr.New()),
	)

Which issue(s) this PR fixes (resolves / be part of):

Other special notes for the reviewers: