[ExpressionLanguage] added ExpressionFunction and ExpressionFunctionProviderInterface
[DependencyInjection] added support for custom expression language functions
[Security] added support for custom expression language functions
[Routing] added support for custom expression language functions
[FrameworkBundle] added a compiler pass for expression language providers
feature #12006 Expression language extensibility (fabpot)

This PR was merged into the 2.6-dev branch.

Discussion
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Expression language extensibility

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10512
| License       | MIT
| Doc PR        | not yet

The way we can add functions to an ExpressionLanguage instance is by using inheritance. #10512 tries to make the expression language in the routing flexible but using inheritance won't work when several bundles want to add functions.

So, this PR takes another approach to solve the problem globally.

Todo:

 * [x] add some more tests
 * [ ] add some docs

Commits
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7c24188 [FrameworkBundle] added a compiler pass for expression language providers
4195a91 [Routing] added support for custom expression language functions
1a39046 [Security] added support for custom expression language functions
79bcd52b [DependencyInjection] added support for custom expression language functions
184742c [ExpressionLanguage] added ExpressionFunction and ExpressionFunctionProviderInterface