add feature flag for including breaking changes regardless of app version

[jkarneges]

Currently, the features to include in a build are determined by the version set in Cargo.toml. For example, setting it to 2.0.0 causes v2 features to be included. However, we want to move to a version management process whereby:

• The version in Cargo.toml in main is set to the most recently released version. Currently that's 1.41.0.
• Building from main can potentially include upcoming breaking changes. For example, if the current version is 1.41.0, and 2.0.0 is imminent, we want a way to include the v2 features by default on main without updating the version yet.

The latter behavior feels a little strange but it is expected with this style of process. Historically we've used a -dev suffix to disambiguate main builds vs release builds but this is not idiomatic.

This PR introduces a breaking-changes feature flag that includes everything, disabled by default. If disabled, only the features appropriate for the version set in Cargo.toml are included, so no change in behavior. However, when a major version change is imminent, we can choose to explicitly enable breaking-changes in Cargo.toml such that building main gets everything.

This PR also sets the version in Cargo.toml to that of the most recent release. This means v2 features will no longer be included by default when building main. Notably, there is no -dev suffix.