refactor: remove dependency on apache http client

[clutcher]

Removing dependency on apache http client verion 4 from eureka-client. Below is copied from #1581 :

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As I see org.apache.httpcomponents:httpclient:4.5.3 is needed only for 3 classes - MonitoredConnectionManager, NamedConnectionPool and SSLSocketFactoryAdapter, which are not used anywhere else in the code. Also from what I see in implementation and current state of spring cloud netflix - it looks like eureka-client is now uses everywhere EurekaHttpClient interface and doesn't not need to provide http client implementation itself.

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• This 3 classes are not really used anywhere in source code and in any of related projects included Spring Cloud ecosystem (checked via search on github and google by full qualified class names).
• httpclient 4 classes are not used anywhere in eureka-client except java doc in EurekaClientConfig
• httpclient 4 classes are used in eureka-client-jersey3 in EurekaJersey3ClientImpl, but implementation are provided by transitive dependency on org.glassfish.jersey.connectors:jersey-apache-connector:3.0.5
• httpclient 4 classes are also used in eureka-core tests in TestableHttpReplicationClient, but implementation are provided by transitive dependency on eureka-test-utils, which depends on eureka-core-jersey3, which depends on org.glassfish.jersey.connectors:jersey-apache-connector:3.0.5

[spencergibb]

While we don't use Apache http client 4 in spring cloud, I can't say what Netflix uses internally. Removing public classes outside a major release is not typically a good thing. Maybe deprecate the classes and make Apache http client 4 optional?

[clutcher]

@spencergibb Not sure how to make it optional in gradle, but I can work on it. Should I get confirmation from someone else if current PR should be remade to make http client 4 optional or we can go with removal?