Add Placeholder setting to TextField

[plamco]

Description

This PR adds a new Placeholder setting to the TextField settings model.

Changes included:

• Added Placeholder property to TextFieldSettings.cs
• Updated TextFieldSettings.Edit.cshtml to expose the placeholder input in the admin UI
• Passed the placeholder value through TextFieldFieldDriver
• Updated editor template to render the placeholder value in the input field

Why:
The placeholder improves the editor experience by giving content authors contextual guidance when entering text values.

Testing

• Configure a TextField inside a Content Type
• Set a Placeholder value
• Create or edit a content item using that type
• Verify that the placeholder appears in the input field

Closes #18562

Video:
https://www.dropbox.com/scl/fi/61roa9rhdm25t292e9kgg/textfield_placeholder.gif?rlkey=f4moandkjkre8g4ofd4f30w9l&dl=0

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[plamco]

@dotnet-policy-service agree

[gvkries]

Please check the other editors for the text field. I think some of them may also benefit from showing the placeholder text (e.g. email, phone, url, etc.).

[sebastienros]

@plamco we would appreciate if you could do the same change on other fields that support this attribute. Maybe also update the existing editors that would support it.

[plamco]

@plamco we would appreciate if you could do the same change on other fields that support this attribute. Maybe also update the existing editors that would support it.

@sebastienros
I scanned all fields in OrchardCore that support an input UI.
The following fields already implement placeholder in a consistent way, so I left them unchanged: LinkField, NumericField.
Boolean fields logically have no placeholder.

These fields currently have hard-coded placeholders or partial implementations, so I refactored them to use the new Settings + Driver + Shape pattern:

- YouTubeField
- TitlePart
- ContentPickerField
- UserPickerField
- TagsField

Let me know if you'd like this PR to also cover any additional fields.

Default value of string.Empty has been added to Placeholder to prevent null reference issues in the UI

Testing / Demo:
A short video demonstrating the placeholders in action has been attached below. It shows how the placeholders appear in the edit forms for all updated fields and parts.

[gvkries]

Remember to include the appropriate editors for each field type.

For example, a text field can have an email editor, which outputs an HTML tag like <input type="email" ... >.
The placeholder text should also appear in this editor.

Editor-specific views follow the naming convention: [FieldType]-[Editor].Edit.cshtml

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