github: implement label functionality

[hyangah]

We plan to classify issues and documents and label them. For GitHub issues, we need to

• post labels: Like internal/commentfix, or internal/related, this can be implemented in a separate package (internal/label or internal/classify).

• retrieve all available labels in the project.

[gopherbot]

Change https://go.dev/cl/634976 mentions this issue: gaby: labels page: skip labels authored by bots

[gopherbot]

Change https://go.dev/cl/635455 mentions this issue: internal/github: operations on repo labels

[gopherbot]

Change https://go.dev/cl/635457 mentions this issue: internal/devtools/labeleval: evaluate labels

[gopherbot]

Change https://go.dev/cl/635855 mentions this issue: internal/labels,labeleval: minor changes

[gopherbot]

Change https://go.dev/cl/635875 mentions this issue: internal/labels: clean issue body

[gopherbot]

Change https://go.dev/cl/635975 mentions this issue: internal/devtools/cmd/labeleval: add more issues

[gopherbot]

Change https://go.dev/cl/635876 mentions this issue: internal/devtools/cmd/labelhist: show label history