[Routing] unify fluent interface in RequestContext
feature #9073 [Routing] unify RequestContext as fluent class (Tobion)

This PR was merged into the 2.6-dev branch.

Discussion
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[Routing] unify RequestContext as fluent class

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | partly
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

- [x] <del>some methods were tagged with `@api` which makes no sense since the class is already tagged as stable. this automatically includes all methods. also our bc promise is only about `@api` on classes</del>
- [x] setParameters the only fluent method! I made them all fluent which makes sense for this class

Commits
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41412f9 [Routing] unify fluent interface in RequestContext