feat: add an opacity slider to map tiles

[KaWaite]

Overview

closes reearth/reearth#278
Users want to be able to set an opacity to map tiles. So, this adds opacity to tiles :P

What I've done

Added slider as an option to schema ui in plugin_manifest
Added Opacity field to tile in manifest w min 0 and max 1, default 1.
Updated appropriate domain models, converters, etc

How I tested

Checked if I could view field in UI, has default and can be changed.

[codecov]

Codecov Report

Merging #138 (df9390b) into main (2c02203) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #138      +/-   ##
==========================================
- Coverage   36.68%   36.68%   -0.01%     
==========================================
  Files         331      331              
  Lines       29888    29890       +2     
==========================================
  Hits        10964    10964              
- Misses      17923    17925       +2     
  Partials     1001     1001              

Impacted Files	Coverage Δ
internal/adapter/gql/gqlmodel/convert_property.go	1.30% <0.00%> (-0.01%)	⬇️
pkg/property/schema_field_ui.go	85.71% <ø> (ø)