feat: support GitHub Enterprise

[suzuki-shunsuke]

#462

Please set the following fields in configuration file github-comment.yaml.

ghe_base_url: https://api.github.com # CHANGE
ghe_graphql_endpoint: https://api.github.com/graphql # CHANGE

• https://docs.github.com/en/enterprise-server@3.5/rest/overview/resources-in-the-rest-api#current-version
• https://docs.github.com/en/enterprise-server@2.20/graphql/guides/forming-calls-with-graphql#the-graphql-endpoint