Introduce ExtensionPointConfiguration

[jlink]

This is an attempt to tackle some of the points discussed in #112 .

Every concrete subtype of ExtensionPoint can now register a configuration of type ExtensionPointConfiguration. For example:

public interface BeforeEachExtensionPoint extends ExtensionPoint...
    /**
     * Configuration for {@code BeforeEachExtensionPoint}
     */
    ExtensionPointConfiguration CONFIG = new ExtensionPointConfiguration(
        new ExtensionPointRegistry.Position[] { OUTERMOST, OUTSIDE_DEFAULT, DEFAULT, INSIDE_DEFAULT, INNERMOST },
        ExtensionPointRegistry.ApplicationOrder.FORWARD);

This configuration is used in ExtensionRegistry two things:

• Determine the application order of an extension point. Formerly that was done by the applier of an extension point who had to know.
• Allow only certain values for Position when registering an extension. All other values will lead to an ExtensionConfigurationException.

The concept has been introduced, because not all values make sense for all types of extension points. For example, ExceptionHandlerExtensionPoint allows FIRST, DEFAULT and LAST.

Extension point types that do not come with their own configuration only allow DEFAULT as position and are thus ordered by their position in the source only.

[codecov-io]

Current coverage is 86.90%

Merging #135 into master will increase coverage by +0.09% as of bf5517e

@@            master   #135   diff @@
=====================================
  Files          134    135     +1
  Stmts         3072   3130    +58
  Branches         0      0       
  Methods        944    963    +19
=====================================
+ Hit           2667   2720    +53
  Partial          0      0       
- Missed         405    410     +5

Review entire Coverage Diff as of bf5517e

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[marcphilipp]

I'm closing this issue since we've decided not to ship any support for extension ordering in M1.