[Ingest Manager] Replace AgentAssetType enum with new approach (#82387)

## Summary

Expands on pattern added in #82188

`AgentAssetType` `enum` is only used as part of a single type definition

Author: John Schulz

x-pack/plugins/ingest_manager/common/constants/epm.ts

@@ -14,6 +14,10 @@ export const requiredPackages = {
   Endpoint: 'endpoint',
 } as const;
 
+export const agentAssetTypes = {
+  Input: 'input',
+} as const;
+
 export const dataTypes = {
   Logs: 'logs',
   Metrics: 'metrics',

x-pack/plugins/ingest_manager/common/types/models/epm.ts

@@ -7,7 +7,8 @@
 // Follow pattern from https://github.com/elastic/kibana/pull/52447
 // TODO: Update when https://github.com/elastic/kibana/issues/53021 is closed
 import { SavedObject, SavedObjectAttributes, SavedObjectReference } from 'src/core/public';
-import { dataTypes, requiredPackages } from '../../constants';
+import { agentAssetTypes, dataTypes, requiredPackages } from '../../constants';
+import { ValueOf } from '../../types';
 
 export enum InstallationStatus {
   installed = 'installed',
@@ -27,7 +28,8 @@ export type EpmPackageInstallStatus = 'installed' | 'installing';
 
 export type DetailViewPanelName = 'overview' | 'usages' | 'settings';
 export type ServiceName = 'kibana' | 'elasticsearch';
-export type AssetType = KibanaAssetType | ElasticsearchAssetType | AgentAssetType;
+export type AgentAssetType = typeof agentAssetTypes;
+export type AssetType = KibanaAssetType | ElasticsearchAssetType | ValueOf<AgentAssetType>;
 
 export enum KibanaAssetType {
   dashboard = 'dashboard',
@@ -47,10 +49,6 @@ export enum ElasticsearchAssetType {
 
 export type DataType = typeof dataTypes;
 
-export enum AgentAssetType {
-  input = 'input',
-}
-
 export type RegistryRelease = 'ga' | 'beta' | 'experimental';
 
 // Fields common to packages that come from direct upload and the registry