[2.3] [Routing] Added access to querystring in RequestContext
merged branch lmcnearney/querystring (PR #7028)

This PR was squashed before being merged into the master branch (closes #7028).

Discussion
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[2.3] [Routing] Added access to querystring in RequestContext

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

This is related to a Silex change request: https://github.com/fabpot/Silex/pull/623

Commits
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4a2b755 [2.3] [Routing] Added access to querystring in RequestContext