Fix Spigot/Paper skin forwarding for Connect tunnels

[robinbraemer]

Summary

• Preserve Connect session profile properties when constructing Spigot authlib GameProfiles so direct Paper/Spigot joins keep player skins.
• Update Paper's PreFillProfileEvent fallback to use real Connect profile properties instead of always inserting an empty textures property.
• Add Spigot tests covering signed texture conversion for authlib and Paper profile properties.

Compatibility notes

• Supports old authlib versions with mutable GameProfile#getProperties().
• Supports newer Paper/authlib record-style GameProfile#properties() and immutable PropertyMap by constructing a populated PropertyMap through reflection.
• Avoids Shadow relocating the server authlib Guava class lookup by building the com.google.common.collect package string dynamically.

Verification

• ./gradlew build
• git diff --check
• Runtime check against Paper 1.21.11 authlib 7.0.61 using the shaded Spigot jar: signed textures property preserved.
• Live comparison with custom skin:
  • Velocity path with Connect on Velocity: skin visible.
  • Direct Paper with released 0.7.4: skin not visible.
  • Direct Paper with this patched Spigot jar: skin visible.