Wrap builtin processors in middleware in examples

[lovromazgon]

Description

This is needed so that the generated specs and consequently the generated documentation contain the middleware parameters.

EDIT: also added the following changes:

• simplified how processor specs are exported (used ENV var instead of build tag, allowing me to remove 8 files)
• disabled schema middleware in error and filter processors (they have no use for it)

Quick checks

• I have followed the Code Guidelines.
• There is no other pull request for the same update/change.
• I have written unit tests.
• I have made sure that the PR is of reasonable size and can be easily reviewed.