Add MergeFrom Methods to MapField for Dictionary and MapField Merging

[RyodoTanaka]

Summary of Changes

This pull request introduces two new methods to the MapField<TKey, TValue> class within the Runtime/Google.Protobuf/Collections/MapField.cs file. These methods enhance the functionality by allowing merging operations.

Changes Introduced

1. Added Method MergeFrom(IDictionary<TKey, TValue> other)

   • This method enables the merging of entries from an IDictionary<TKey, TValue> into the current MapField instance.
   • It iterates through the provided dictionary (other) and updates the current map with its key-value pairs.

2. Added Method MergeFrom(MapField<TKey, TValue> other)

   • This method allows the merging of entries from another MapField<TKey, TValue> instance.
   • Internally, it reuses the MergeFrom(IDictionary<TKey, TValue>) implementation for consistency.

Benefits

• Simplifies the process of merging data into MapField instances.
• Improves code reusability by leveraging a shared implementation for both IDictionary and MapField.

Code Changes

Below is a snippet of the added code:

/// <summary>
/// Merges entries from an <see cref="IDictionary{TKey, TValue}"/> into this instance.
/// </summary>
/// <param name="other">
/// The dictionary whose entries will be merged into this map.
/// </param>
public void MergeFrom(IDictionary<TKey, TValue> other)
{
    foreach (var kv in other)
    {
        this[kv.Key] = kv.Value;
    }
}

/// <summary>
/// Merges entries from another <see cref="MapField{TKey, TValue}"/> into this instance.
/// </summary>
/// <param name="other">
/// The <see cref="MapField{TKey, TValue}"/> whose entries will be merged.
/// </param>
public void MergeFrom(MapField<TKey, TValue> other)
{
    // Reuse the IDictionary<TKey, TValue> MergeFrom implementation
    MergeFrom((IDictionary<TKey, TValue>)other);
}