Blackholio - Hide generated files from diffs (#3119)

# Description of Changes

Just add a `.gitattributes` file that hide Blackholio autogenerated
files from GitHub diffs

# API and ABI breaking changes

None

# Expected complexity level and risk

1

# Testing

- [x] If I change one of the autogenerated files, it's hidden:
https://github.com/clockworklabs/SpacetimeDB/pull/3119/files/8601b4d5316c66f91a2555e720fcfeb01b3241db

---------

Co-authored-by: Zeke Foppa <bfops@users.noreply.github.com>

Author: bfops

demo/Blackholio/client-unity/.gitattributes

@@ -0,0 +1 @@
+/Assets/Scripts/autogen/** linguist-generated=true