feat: add totalCount field to DatasetSchema type of GraphQL schema

[keiya01]

Overview

I fixed reearth/reearth-visualizer#267.

What I've done

• add CountBySchema method to count dataset by schema id.
• add totalCount resolver to DatasetSchema struct

What I haven't done

How I tested

1. run reearth-backend and reearth-web
2. add dataset
3. check dataset count

Which point I want you to review particularly

Memo

[codecov]

Codecov Report

Merging #154 (cebf885) into main (e444e4e) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #154      +/-   ##
==========================================
- Coverage   34.84%   34.81%   -0.04%     
==========================================
  Files         351      351              
  Lines       31868    31901      +33     
==========================================
  Hits        11105    11105              
- Misses      19751    19784      +33     
  Partials     1012     1012              

Impacted Files	Coverage Δ
internal/adapter/gql/loader_dataset.go	0.00% <0.00%> (ø)
internal/adapter/gql/resolver_dataset_schema.go	0.00% <0.00%> (ø)
internal/infrastructure/memory/dataset.go	0.00% <0.00%> (ø)
internal/infrastructure/mongo/dataset.go	0.00% <0.00%> (ø)
internal/usecase/interactor/dataset.go	0.00% <0.00%> (ø)
internal/usecase/repo/dataset.go	0.00% <ø> (ø)

[keiya01]

I ran go generate ./... to generate some gql code, but this generated too many code in this commit.
chore: generate gql

Is this way correct?
Or is there another way to generate gql code?