Move generated promo step type definitions to pkg

[jessesuen]

Proposed Feature

Currently the golang versions of native promotion steps are auto generated to the internal/directives directory:
https://github.com/akuity/kargo/blob/main/internal/directives/zz_config_types.go

For example:

type GitCloneConfig struct {
	// The commits, branches, or tags to check out from the repository and the paths where they
	// should be checked out. At least one must be specified.
	Checkout []Checkout `json:"checkout"`
	// Indicates whether to skip TLS verification when cloning the repository. Default is false.
	InsecureSkipTLSVerify bool `json:"insecureSkipTLSVerify,omitempty"`
	// The URL of a remote Git repository to clone. Required.
	RepoURL string `json:"repoURL"`
}

We should move these outside of internal into pkg so they can be publically consumed.

Motivation

Anyone who is programmatically construct a Kargo object from its CRD types, or the reverse: unmarshalling them into an golang object, do not have access to these types since they are in the internal directory.

Suggested Implementation

Change the path where these are generated and the corresponding import statements.