feat: support resolver-backed COPC conversion

[snkmcb]

Summary

• accept resolver-addressable COPC identifiers in usd-pointcloud-convert
• reuse and publish generated cache entries only for Stable resolver identity
• keep resolver identifiers and validation tokens out of manifests
• add resolver miss-to-hit and unstable-identity tests
• add the COPC OST smoke fixture and synchronize the documentation

Validation

• usdPointCloudConvert_las
• usdPointCloudConvert_copc
• usdPointCloudConvert_resolver_copc
• pointcloudCopc_integration
• ost plugin test .\plugins\pointcloud-copc --up-to 4
• git diff --check