feat: add missing extension YAMLs and test for completeness

[benbellick]

Closes #722

Adds functions_set to DefaultExtensionCatalog and a test that dynamically discovers all extension YAMLs in substrait/extensions/ and verifies they are loaded, so future omissions are caught automatically.

Files not yet loaded are listed in the test's UNSUPPORTED_FILES set with inline comments explaining why.

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[benbellick]

This was rebased off of #726, so I will wait for that one to be merged before merging this one.

[benbellick]

As noted when functions_aggregate_decimal_output was added, it is causing problems for substrait-java. So, I have continued to skip it in this PR, but made that skipping explicit.

[nielspardon]

I'm wondering whether this should check the extension YAML files on the classpath instead of relying on the current working directory (especially given that @vbarua is planning to remove the submodule and move packaging the extension YAMLs into a separate JAR into substrait-packaging). The YAMLs should be already on the classpath. We're already loading classgraph for the isthmus-cli which can be useful to scan the classpath for any *.yaml files.

[benbellick]

Sounds good to me, I'll make that change shortly. Thanks!

[nielspardon]

btw. it might be better to relocate the YAML files on the classpath into a subfolder like substrait/extensions to be easier recognizable as default YAML files. right now they are being added into the root of the classpath