prepare 3.0.0 release

[LaunchDarklyReleaseBot]

[3.0.0] - 2022-11-30

This major version release of go-sdk-common corresponds to the upcoming v6.0.0 release of the LaunchDarkly Go SDK (go-server-sdk), and cannot be used with earlier SDK versions.

Added:

• The new package ldcontext with the types Context and Kind defines the new "context" model. "Contexts" are a replacement for the earlier concept of "users"; they can be populated with attributes in more or less the same way as before, but they also support new behaviors. More information about these features will be included in the release notes for the v6.0.0 SDK release.
• The new package ldattr defines the attribute reference syntax, for referencing subproperties of JSON objects in flag evaluations or private attribute configuration. Applications normally will not need to reference this package.

Changed:

• The minimum Go version is now 1.18.
• The SDK packages now use regular Go module import paths rather than gopkg.in paths: gopkg.in/launchdarkly/go-sdk-common.v2 is replaced by github.com/launchdarkly/go-sdk-common/v3.
• The type lduser.User has been redefined to be an alias for ldcontext.Context. This means that existing application code referencing lduser.User can still work as long as it is treating the user as an opaque value, and not calling methods on it that were specific to that type.
• lduser.NewUser and lduser.UserBuilder now create an instance of Context instead of User. This is as a convenience so that any code that was previously using these methods to construct a user, but did not reference the User type directly for the result, may still be usable without changes. It is still preferable to use the new constructors and builders for Context.
• The Secondary attribute which existed in User does not exist in Context and is no longer a supported feature.
• It was previously allowable to set a user key to an empty string. In the new context model, the key is not allowed to be empty. Trying to use an empty key will cause evaluations to fail and return the default value.
• If you were using JSON serialization to produce a representation of a User, the new type Context uses a different JSON schema, so any code that reads the JSON will need to be adjusted. If you are passing the JSON to other code that uses LaunchDarkly SDKs, make sure you have updated all SDKs to versions that use the new context model. (However, unmarshaling a Context from JSON data will still work correctly even if the JSON is in the old user format.)

Removed:

• Removed the Secondary meta-attribute in lduser.UserBuilder.