When running `ng update` we now display optional migrations from packages.

When the terminal is interactive, we prompt the users and ask them to choose which migrations they would like to run.
```
$ ng update @angular/core --from=14 --migrate-only --allow-dirty
Using package manager: yarn
Collecting installed dependencies...
Found 22 dependencies.
** Executing migrations of package '@angular/core' **

▸ Since Angular v15, the `RouterLink` contains the logic of the `RouterLinkWithHref` directive.
  This migration replaces all `RouterLinkWithHref` references with `RouterLink`.
  Migration completed (No changes made).

** Optional migrations of package '@angular/core' **

This package have 2 optional migrations that can be executed.
Select the migrations that you'd like to run (Press <space> to select, <a> to toggle all, <i> to invert selection, and <enter> to proceed)
❯◯ Update server builds to use generate ESM output.
 ◯ Lorem ipsum dolor sit amet, consectetur adipiscing elit.
```

In case the terminal is non interactive, we will print the commands that need to be executed to run the optional migrations.
```
$ ng update @angular/core --from=14 --migrate-only --allow-dirty
Using package manager: yarn
Collecting installed dependencies...
Found 22 dependencies.
** Executing migrations of package '@angular/core' **

▸ Since Angular v15, the `RouterLink` contains the logic of the `RouterLinkWithHref` directive.
  This migration replaces all `RouterLinkWithHref` references with `RouterLink`.
  Migration completed (No changes made).

** Optional migrations of package '@angular/core' **

This package have 2 optional migrations that can be executed.

▸ Update server builds to use generate ESM output.
  ng update @angular/core --migration-only --name esm-server-builds

▸ Lorem ipsum dolor sit amet, consectetur adipiscing elit.
  ng update @angular/core --migration-only --name migration-v15-router-link-with-href
```

**Note:** Optional migrations are defined by setting the `optional` property to `true`. Example:
```json
{
  "schematics": {
    "esm-server-builds": {
      "version": "15.0.0",
      "description": "Update server builds to use generate ESM output",
      "factory": "./migrations/relative-link-resolution/bundle",
      "optional": true
    }
}
```

Closes angular#23205