feat(console): replace target column with kind column in tasks view

[hds]

In the tokio-console tasks view, there are a fixed set of columns and
any remaining fields are included in a "Fields" column at the end. One
of the fields which is always present on a task, but doesn't receive a
dedicated column is the kind field, which currently takes one of the
following values:

• task (a "normal" async task)
• blocking
• block_on
• local

Meanwhile, there is a dedicated column for the task span's target, which
currently takes one of the following values:

• tokio::task
• tokio::task::blocking

The target for tasks with kind block_on and local is also
tokio::task.

This change replaces the target column with a kind column as it provides
more information in fewer characters. The target value is moved
(somewhat artificially) to the fields which appear in the final column.

The target is also left on the state::Task struct as we expect to
want to filter by it in the future.

Additionally, the console-subscriber examples have been updated so
that there are options to visualize blocking, block_on, and local
tasks. The app example has been updated to include an optional task
which calls tokio::spawn_blocking. A new example local has been
added which creates a LocalSet and spawns local tasks onto it.