Update actions fields and methods

[alexkappa]

Proposed Changes

• Modify ActionManager methods to adhere to conventions of the SDK.
• Add Runtime field to Action
• Add comments to fields.

Acceptance Test Output

$ go test ./... -run TestAction                                                                                                                                                                                                                                          [17:35:27]
ok      gopkg.in/auth0.v5       (cached) [no tests to run]
ok      gopkg.in/auth0.v5/internal/client       (cached) [no tests to run]
ok      gopkg.in/auth0.v5/internal/tag  (cached) [no tests to run]
ok      gopkg.in/auth0.v5/internal/testing/expect       (cached) [no tests to run]
ok      gopkg.in/auth0.v5/management    8.390s

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