Replace `InitValueTermOptions` method with `GetValueTermOptions` in `Field<T, TOwner>`

[YevgeniyShunevych]

• Remove protected virtual void InitValueTermOptions(TermOptions termOptions, UIComponentMetadata metadata) method.
• Add protected virtual TermOptions GetValueTermOptions() method.

Migration to v2.0.0

Before:

protected override void InitValueTermOptions(TermOptions termOptions, UIComponentMetadata metadata)
{
    base.InitValueTermOptions(termOptions, metadata);

    termOptions.MergeWith(SelectOptionBehavior);
}

After:

protected override TermOptions GetValueTermOptions() =>
    base.GetValueTermOptions().MergeWith(SelectOptionBehavior);