support override java agent jar.

[JaredTan95]

Fixes #243.

Changes

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• Appropriate CHANGELOG.md updated for non-trivial changes
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[puckpuck]

This can be done as a separate PR. We should also update how the Agent's jar file is sourced. Right now the binary is part of this repo, but really we should be pulling it from the Java instrumentation releases instead. ie: https://github.com/open-telemetry/opentelemetry-java-instrumentation/releases/download/v1.16.0/opentelemetry-javaagent.jar

[JaredTan95]

but really we should be pulling it from the Java instrumentation releases instead

okay, I will update it with a separate PR soon.