`InfoProvider` is now `InfoContributor` and contributes to the `Info`
instance via a builder. The `Info` instance is immutable. Each
contributor can be disabled via the `management.info.<name>.enabled`
key or all can be disabled using `management.info.defaults.enabled` (this
is similar to what the health endpoint does).

By default, all keys from the environment starting with `info.` are
exposed. If a `git.properties` file is present in the classpath, the
content of `GitInfo` is exposed using the `git` key.

A `SimpleInfoContributor` and `AbstractEnvironmentInfoContributor` are
available for convenience. `InfoContributor` instances can be ordered
the usual way, with a default order provided by
`InfoProviderAutoConfiguration#DEFAULT_ORDER`.

Closes gh-3492