Generate nicer SDK getters

[johanbrandhorst]

Is your feature request related to a problem? Please describe.
When using the SDK, the result getters all return interface{} when they have a concrete type available. This is a poor experience for the user. See

boundary/api/sessions/session.gen.go

Lines 45 to 47 in 2a5aaea

	func (n SessionReadResult) GetItem() interface{} {
	return n.Item
	}

and

boundary/api/sessions/session.gen.go

Lines 74 to 76 in 2a5aaea

	func (n SessionListResult) GetItems() interface{} {
	return n.Items
	}

for examples.

Describe the solution you'd like
I would like the generated code to use typed returns.

Additional context
This was raised as a concern by internal users of our SDK. The template for this part of the generated SDK is here:

boundary/internal/api/genapi/templates.go

Lines 564 to 631 in 2a5aaea

	var structTemplate = template.Must(template.New("").Parse(
	fmt.Sprint(`// Code generated by "make api"; DO NOT EDIT.
	package {{ .Package }}
	import (
	"context"
	"fmt"
	"time"
	"github.com/kr/pretty"
	"github.com/hashicorp/boundary/api"
	"github.com/hashicorp/boundary/api/scopes"
	)
	type {{ .Name }} struct { {{ range .Fields }}
	{{ .Name }} {{ .FieldType }} `, "`json:\"{{ .ProtoName }},omitempty\"`", `{{ end }}
	{{ if .CreateResponseTypes }}
	response *api.Response
	{{ else if ( eq .Name "Error" ) }}
	response *Response
	{{ end }}
	}
	{{ if .CreateResponseTypes }}
	type {{ .Name }}ReadResult struct {
	Item *{{ .Name }}
	response *api.Response
	}
	func (n {{ .Name }}ReadResult) GetItem() interface{} {
	return n.Item
	}
	func (n {{ .Name }}ReadResult) GetResponse() *api.Response {
	return n.response
	}
	type {{ .Name }}CreateResult = {{ .Name }}ReadResult
	type {{ .Name }}UpdateResult = {{ .Name }}ReadResult
	type {{ .Name }}DeleteResult struct {
	response *api.Response
	}
	// GetItem will always be nil for {{ .Name }}DeleteResult
	func (n {{ .Name }}DeleteResult) GetItem() interface{} {
	return nil
	}
	func (n {{ .Name }}DeleteResult) GetResponse() *api.Response {
	return n.response
	}
	type {{ .Name }}ListResult struct {
	Items []*{{ .Name }}
	response *api.Response
	}
	func (n {{ .Name }}ListResult) GetItems() interface{} {
	return n.Items
	}
	func (n {{ .Name }}ListResult) GetResponse() *api.Response {
	return n.response
	}
	{{ end }}
	`)))

[covetocove]

Thank you for raising this item @johanbrandhorst! We'll keep this item open to gauge interest from the community to inform its priority on our roadmap.

[jefferai]

Closing as #2238 has been merged