Bump version to v27

Add methods for GitHub Enterprise endpoints (google#1214)

Add repository creation permissions field (google#1213)

Added Support preview Team Sync API for GitHub Enterprise Cloud users (…

…google#1212)

Fixes google#1200.

Consistent errors for GetRef when a Ref doesn't exist (google#1207)

Fixes google#1208.

Add Visibility to UserEmail (google#1215)

Add missing `LdapDn` field in Users class (google#1211)

Fixes google#1210 for GitHub Enterprise users.

Add missing RepositoryComment.NodeID field. (google#1209)

Add SignKey GPG entity param to allow easier signing of commits (goog…

…le#1198)

Add gen-stringify-test.go (google#1196)