* GH-3154: Support `UriBuilderFactory.EncodingMode`
Fixes https://github.com/spring-projects/spring-integration/issues/3154
Spring Framework now provides a `DefaultUriBuilderFactory.EncodingMode`
for encoding URIs in the `RestTemplate` before and after uri template
enrichment with uri variables.
Therefore `encodeUri` and manual uri variables substitution is not necessary
in Spring Integration HTTP components
* Deprecate `AbstractHttpRequestExecutingMessageHandler.encodeUri` in favor of
`DefaultUriBuilderFactory.EncodingMode` and respective configuration
on the `RestTemplate` in HTTP module and `WebClient` in WebFlux module
* * Really populate `uriFactory` into an internal `RestTemplate`
* Ensure in tests that `encoding-mode` is populated properly into an internal `RestTemplate`
* Clean up affected HTTP tests for AssertJ and JUnit 5
* * Clean up formatting
* * Apply fix for WebFlux module
* Add docs for new `encoding-mode` option
* * Remove unused import in the test
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