Switch RestServiceHandler from RestTemplate to RestClient.

[paveljandejsek]

#883
RestServiceHandler is a mess at this moment so this is just a non-ideal minimalistic change (Ideally we would not even create RequestEntity, instead of current creating it and then using body and headers from it...)
Considerations:

• renaming the initTemplate to initRestClient, removing RestTemplateConfigurer - lots of breaking changes sadly 🙁
• by keeping the old naming pattern we are still leaking implementation details
• should we use autoconfigured RestClientBuilder/RestClient instead of creating our own? (I would almost say so, but this PR contains the same "old approach")
• instead of clientConfigurer (aka old templateConfigurer) naming I used the fuller restClientConfigurer
• proposed oauth2 support added in separate commit - without any test configuration for now, will need to be added along with some test (ideally once we agree that this is how we should implement it or decide how else to implement it)