Polish contribution

This commit polishes the HTTPie request snippet contribution made in
b26d8c0. It makes the following significant changes:

- Applies project’s coding conventions for formatting and the like
- Moves to a composition-based approach for sharing functionality
  between the curl and HTTPie snippets by replacing AbstractCliSnippet
  with CliOperationRequest.
- Introduces a single package for CLI command snippets, thereby allowing
  more code to be package-private.

See gh-207

spring-restdocs-core/src/test/resources/custom-snippet-templates/asciidoctor/httpie-request-with-title.snippet

@@ -1,5 +1,5 @@
 [source,bash]
 .{{title}}
 ----
-$ {{echo_content}}http {{options}} {{url}}{{request_items}}
+$ {{echoContent}}http {{options}} {{url}}{{requestItems}}
 ----

spring-restdocs-core/src/test/resources/custom-snippet-templates/markdown/httpie-request-with-title.snippet

@@ -1,4 +1,4 @@
 {{title}}
 ```bash
-$ {{echo_content}}http {{options}} {{url}}{{request_items}}
+$ {{echoContent}}http {{options}} {{url}}{{requestItems}}
 ```