Adapt plugincache MergeStrategy to sbt-assembly:2.0.0

[fnqista]

Update MergeStrategy code to use temp files to extract the InputStreams
Update the sbt assembly plugin to use 2.0.0-RC1
Update the log4j version to 2.17.2
Update version to 2.1.0

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Hey maintainers,

We need to update this for the upcoming sbt-assembly 2.0.0 (right now it's in RC1 mode)

Background

sbt-assembly has recently been updated to use a streaming/in-memory model instead of IO/disk-extraction model. Due to this, the method signature of the MergeStrategy has changed.

I also went ahead and updated the log4j dependency to 2.17.2 to fix that infamous vulnerability.

[mpollmeier]

Thank you for this PR. The project seems unmaintained, so I took the liberty to fork it, include your PR, upgrade everything and release it to maven central. I added you to the https://github.com/mpollmeier/sbt-assembly-log4j2#credits and hope that's ok?

https://github.com/mpollmeier/sbt-assembly-log4j2