Connect custom compression to crash test and ObjectLibrary

[pdillinger]

Summary: Some pieces of follow-up to #13659.
Recommend hiding whitespace for review

• Add support for instantiating CompressionManagers through CreateFromString/ObjectLibrary.
• Pull CompressorCustomAlg and DecompressorCustomAlg out of db_test2, refactor/improvement them a bit, and put them in testutil.h for sharing with db_stress. Switched it from being built on snappy to being built on lz4 so that it can properly test dictionary compression.
• Add a custom compression manager for db_stress that uses these, and add to crash test. This depends on the ObjectLibrary stuff because some invocations of db_stress will not be configured with the custom compression manager but will need to access it to read some existing SST files.
• Remove some pieces where the concern of setting compression=kZSTD for compatibility purposes had leaked into configuring some tests and compression managers. After Preliminary support for custom compression algorithms #13659 this compatibility concern is contained in the SST building code.

Test Plan: unit tests updated

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[hx235]

Thanks!