KAFKA-12357: Do not inline methods from the scala package by default

[ijuma]

As mentioned in #9548, users currently use the kafka jar (core module)
for integration testing and the current inlining behavior causes
problems when the user's classpath contains a different Scala version
than the one that was used for compilation (e.g. 2.13.4 versus 2.13.3).

An example error:

java.lang.NoClassDefFoundError: scala/math/Ordering$$anon$7

We now disable inlining of the scala package by default, but make it
easy to enable it for those who so desire (a good option if you can
ensure the scala library version matches the one used for compilation).
While at it, we make it possible to disable scala compiler optimizations
(none) or to use only method local optimizations (method). This can
be useful if optimizing for compilation time during development.

Verified behavior by running gradlew with --debug and checking the
output after [zinc] The Scala compiler is invoked with:

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[ijuma]

@chia7712 I was thinking to include this in 2.8 since it's low risk and it fixes a regression when it comes to writing integration tests with Kafka.

[chia7712]

overall +1.

one question:
As we don't guarantee to keep core compatibility, is it worthwhile to exclude scala inlining by default?

[ijuma]

@chia7712 For better or worse, libraries like https://github.com/embeddedkafka/embedded-kafka are often used. And there isn't a great alternative. Getting an exception when there's a mismatch in the patch version of scala-library is pretty surprising. It seems better to aim for ease of use until there's a better way of handling this. Do you agree?

[chia7712]

It seems better to aim for ease of use until there's a better way of handling this

make sense. be a friendly libraries :)

[ijuma]

Merged to trunk and 2.8 branches.