Plan for serde_dynamo 4

[bryanburgers]

The plan for serde_dynamo version 4 will be:

• Support aws-sdk version 1.0
• Drop support for rusoto

serde_dynamo version 3 (published today) effectively supports both aws-sdk and rusoto based on feature flags. It does this because neither of those libraries has reached 1.0; without feature-flag support, any minor version bump to those libraries would require a major version bump to serde_dynamo.

In version 4, we plan to drop support for rusoto. Rusoto is currently in "maintenance mode", so version 3 will continue to support it and there is no reason to continue to support it into version 4 and beyond.

When aws-sdk reaches version 1.0, we can drop the feature flags and just support version 1.0 because at that point, minor bumps to aws-sdk version 1.0 will not require a re-release of serde_dynamo.

[bryanburgers]

Change of plans.

• I don't think the community will coalesce around one single definition of AttributeValue. I was hoping that aws_sdk_dynamodb::model::AttributeValue would be the AttributeValue, but even aws_sdk_dynamodbstreams won't adopt it as the de-facto definition.
• The AttributeValue trait definition in v3, as defined, does not work aws_lambda_event's AttributeValue. That makes it hard to complete Deserialize DDB item from JSON #24 and Add Support for aws_lambda_event's AttributeValue #35.
• The AttributeValue trait definition in v3 also caused the compile time of one of Zenlist's key projects to balloon from 20 minutes to 40 minutes, so we never upgraded from v2 to v3.

Given all of that, I looked into a new way to approach this library, and settled on the following.

• serde_dynamo v4 will define it's own AttributeValue enum.
• serde_dynamo v4 will implement From<external_crate::AttributeValue> for AttributeValue and From<AttributeValue> for external_crate::AttributeValue based on features, to make it easy to convert between e.g. aws_sdk_dynamodb::model::AttributeValue and serde_dynamo::AttributeValue.
• Functions like from_attribute_value/to_attribute_value will deal with anything that is From<AttributeValue>/Into<AttributeValue> to provide an API that is almost exactly like the v3 API.

[mbergkvist]

This "new type" approach is similar approach I took in my project to convert from the aws_lambda_event AttributeValue to aws_sdk AttributeValue, which I could then use with serde_dynamo 3.