[1.x] Backporting #19016

[samskalicky]

Description

Partially backporting #19016:

• Refactors lib_api.h by moving the function definitions into lib_api.cc to simplify building extensions
• Updates all the examples in example/extensions to use the new lib_api.cc build flow and updates version numbers in examples to 1.8

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented

Changes

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here

[mxnet-bot]

Hey @samskalicky , Thanks for submitting the PR
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[samskalicky]

@rondogency do you wanna change this description in the header/cc files?
https://github.com/apache/incubator-mxnet/pull/19069/files#diff-5c218b5f3e585d7cc7e4ca8b88470d40R23-R26

[mseth10]

LGTM