[ios][secure_paste]Show context items based on framework's data

[hellohuanlin]

This PR shows the context menu based on what's received from the framework via method channel. There are 2 cases:

1. For basic editing actions (e.g. copy/paste):

We perform a DFS search inside suggestedActions, so that we don't have to specify things like keyboard shortcut.

2. For additional actions (e.g. look up, share, search web):

We manually create UICommand because (1) some actions are not provided by suggestedActions (2) these are simply UICommand (rather than UIKeyCommand), so we just need title and action (3) the actions are private APIs. However this would require framework to send the item title.

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Second milestone for flutter/flutter#103163

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[hellohuanlin]

@justinmc This PR is not complete, but it should be good enough to start the framework side on top of it.

Example usage:

SystemChannels.platform.invokeMethod<void>(
      'ContextMenu.showSystemContextMenu',
      <String, dynamic>{
        'targetRect': <String, dynamic>{
          'x': 100,
          'y': 100,
          'width': 100,
          'height': 100,
        },
        'items': <dynamic>[
          <String, dynamic>{
            'type': 'default',
            'action': 'paste',
          },
          <String, dynamic>{
            'type': 'default',
            'action': 'cut',
          },
          <String, dynamic>{
            'type': 'default',
            'action': 'copy',
          },
          <String, dynamic>{
            'type': 'default',
            'action': 'selectAll',
          },
          <String, dynamic>{
            'type': 'default',
            'title': 'Look Up',
            'action': 'lookUp',
          },
          <String, dynamic>{
            'type': 'default',
            'title': "Search Web",
            'action': 'searchWeb',
          },
          <String, dynamic>{
            'type': 'default',
            'title': "Share",
            'action': 'share',
          },
        ],
      }
    );

I haven't implemented the additional actions (e.g. look up), so currently simply print out a log.

Also notice that I added title field for the additional actions. You can decide either add title to all actions for consistency, or just the additional actions.