REVIEW: [NEXUS-5214] Make possible to configure separate general (default) HTTP proxies for http and https

https://issues.sonatype.org/browse/NEXUS-5214
https://issues.sonatype.org/browse/NEXUS-5690
https://bamboo.zion.sonatype.com/browse/NXO-OSSF61

The changes required:
* HC4: Hc4ProviderBase, NonProxyHostsAwareHttpRoutePlanner, UrlSchemeAwareHttpRoutePlanner 97a04dc0cab950dca3aa529947cc94995325378f
* nexus configuration to accomodate the new globalHttpsProxySettings  section 9bffe21dd0e8800cac35
* add GlobalHttpsProxySettings that is the iface to interact with nexus config regarding the new configuration option. As this is similar to GlobalHttpProxySettings, in order to reuse code I extracted superclasses and inherit them in both http/https 97a04dc0cab950dca3aa529947cc94995325378f and  3e3bc028dc4edce1b41f70c6cb8dcf0df45344cc
* REST (value objects/resource) d44da1738b223427137bdca04ec72781ab305807
* UI 22f088b5c1b63980f50d044d44d3ade52b72acfd and 9ba3e2273e5358475a1df042bfe80a25d0965a5b
* IT testing what proxy is used based on combination of defined proxies at global http/https/repository level and used url scheme (http/https) 01c64f719158b966dadd492b54f97282b03e5f67
* add support for setting https proxy via nexus client b88d07d8428b9a7dcc49564f9da52802ce444678
* add support for setting web proxy on repository via nexus client 1d994349f5de779b2ee93f64b2120cbaa9259da8