Cosmos: support for mapping ReadOnlyMemory and/or similar

[ajcvickers]

Default mapping would be to to a JSON array of the scalar type. However, we may also want to support Base64 encoding.

Note that this is a bit different to supporting the type in relational, where we agreed that the ADO.NET layer should be responsible, and Npgsql already supports this. For Cosmos, there really isn't any type mapping at the SDK level, so I think it makes sense to support it directly in the EF provider.