Move annotation discovery into AbstractEntryBasedExtension

[beatngu13]

Closes #460. This issue was basically a description of the current usage of both BeforeAllCallback and BeforeEachCallback, which was indirectly followed by a discussion on "Enclosing classes vs Before/AfterAll" in #479. As a goal in the first issue, I said I would like to add an implementation note to prevent further confusion. In addition, it would be helpful if a core part of the implementation – annotation discovery – was a single point of failure, and not duplicated in EnvironmentVariableExtension and SystemPropertyExtension.

Note that this PR is based on the currently unmerged changes in #481. I also marked this as a draft since I haven't updated the README.md yet to not cause merge conflicts.

Proposed commit message:

Move annotation discovery into AbstractEntryBasedExtension (#460 / #485)

This PR removes the duplicated annotation discovery in
`EnvironmentVariableExtension` and `SystemPropertyExtension` and moves
it into `AbstractEntryBasedExtension`.

Closes: #460
PR: #485

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[nipafx]

Looks good to me. The annotations as generic types and the ...Mapper methods are a bit weird at first sight, but all together they make for a pretty type-safe way to handle the problem, particularly given annotations can't implement interfaces.