chore: Update admin build/serve configuration (#9584)

**Breaking changes**

The `outDir` has been deprecated and wont be used anymore, instead all the path are computed internally following these rules
- if admin is not `disabled` and the `build` command is run without the `--admin-only` flag, then the admin output dir will be `.medusa/server/public/admin` and it will be served from that same location from the medusa instance.
- if admin is not `disabled` and the `build` command is run with the `--admin-only` flag, then the admin output dir will be `.medusa/admin` with the purpose of deploying the admin separately. ⚠️ (expect to receive a warning log)
- if the admin is `disabled` and the `build` command is run with the `--admin-only` flag, then fallback to rule number 2

| admin enabled  | medusa build --admin-only  | output dir |
|---|---|---|
| true  | true  | `.medusa/admin` ⚠️ (expect to receive a warning log) |
| true  | false  | `.medusa/server/public/admin`  |
| false  | true  | `.medusa/admin`  |
| false  | false  | none |

```diff
// medusa-config.ts

{
  // ...
  admin: {
-    outDir: 'some/path'
  }
}
```


cc @kasperkristensen @sradevski @olivermrbl

packages/medusa/src/utils/admin-consts.ts

@@ -0,0 +1,3 @@
+export const ADMIN_SOURCE_DIR = "src/admin"
+export const ADMIN_RELATIVE_OUTPUT_DIR = "./public/admin"
+export const ADMIN_ONLY_OUTPUT_DIR = ".medusa/admin"

packages/medusa/src/utils/index.ts

@@ -2,3 +2,4 @@ export * from "./clean-response-data"
 export * from "./exception-formatter"
 export * from "./middlewares"
 export * from "./define-middlewares"
+export * from "./admin-consts"