Support deserialization of immutable dictionaries from avro maps

[kingwill27]

Adds special handling for immutable dictionaries, which cannot be directly assigned to from dictionaries.

[dstelljes]

Thanks for this! Just one small change and I think we’re good to go.

In the future, we may also want to look at altering BinaryCodec so that we aren’t creating both a Dictionary<K, V> and an ImmutableDictionary<K, V> each time.

[dstelljes]

in the array case, we override GenerateConversion: https://github.com/ch-robinson/dotnet-avro/blob/development/src/Chr.Avro.Binary/BinaryDeserializerBuilder.cs#L468

for consistency, it’s probably best to do the same here

[kingwill27]

@dstelljes thanks for the feedback! let me know if that's what you had in mind. I agree it's not ideal to create an intermediate dictionary, although from what I've seen there isn't a great way to construct immutable dictionaries from lists or other data structures.