provide sync method in RoleService instead

[WindzCUHK]

athenz-client-sidecar/service/role.go

Lines 44 to 49 in 3b00963

	// RoleService represents an interface to automatically refresh the role token, and a role token provider function pointer.
	type RoleService interface {
	StartRoleUpdater(context.Context) <-chan error
	RefreshRoleTokenCache(ctx context.Context) <-chan error
	GetRoleProvider() RoleProvider
	}

follow the interface in authorizer
https://github.com/AthenZ/athenz-authorizer/blob/78de9daaa7d5c15f191af8f6af5eb915bcb04035/policy/daemon.go#L39-L46

Suggestion:

type RoleService interface {
	Start(context.Context) <-chan error
	Update(context.Context) error
	GetRoleProvider() RoleProvider
}

migrated from: yahoojapan/athenz-client-sidecar#20