refactor: use resource library.Duration() functions

[jbrockopp]

Based off of go-vela/types#222 and go-vela/types#227

Similar to go-vela/cli#333

This change uses the <resource>.Duration() functions from the github.com/go-vela/types/library package:

• https://pkg.go.dev/github.com/go-vela/types/library#Build.Duration
• https://pkg.go.dev/github.com/go-vela/types/library#Service.Duration
• https://pkg.go.dev/github.com/go-vela/types/library#Step.Duration

Also included in this change is the removal of the duration() helper functions for each resource.