`core` improve `FeignUtil` `BaseBuilder` to allow the client to choose the `Feign` logger as well as the `Feign` `LogLevel`

[francoisledroff]

improve FeignUtil BaseBuilder to allow the client to choose

• its Feign logger
• as well as the Feign LogLevel

see #145 (comment)

It looks like the Feign Logger API is under refactoring see

• https://github.com/OpenFeign/feign#short-term---what-were-working-on-now-
• https://github.com/OpenFeign/feign/issues?q=is%3Aissue+is%3Aopen+logger+label%3Afeign-12
• Use SLF4J for library logging. OpenFeign/feign#1143

a few Feign logging related articles:

• https://cloud.spring.io/spring-cloud-openfeign/reference/html/#feign-logging
• https://medium.com/@piszu/feign-client-is-not-so-simple-as-we-want-case-study-920e4901e27b