This commit adds a new plugin interface to qiskit for enabling external
packages to write plugins that will replace a stage in the transpilation
pipeline. For example, if an external package had a custom layout pass
that they wanted to integrate into transpile() they could export a
plugin using this new interface and then users would just need to run

transpile(.., layout_method=foo)

This adds long asked for extensibility to the transpiler so that to
cleanly integrate new transpiler passes we're no longer required to
merge the features into terra. This should hopefully make it easier for
downstream pass authors to integrate their passes into terra and make it
easier for the terra maintainers to evaluate new transpiler passes.

This commit converts all the built-in routing method options into
separate plugins and also adds a default plugin for the default behavior
at each optimization level. To support using plugins for routing method
adding the optimization_level to the passmanager config was necessary so
that the plugin has sufficient context on how to construct the routing
pass used for the routing stage. As depending on the optimization level
the settings for each pass differs. For example on stochastic swap the
number of stochastic trials increases for level 3 to try and find a
better solution at the cost of more runtime.

Co-authored-by: Alexander Ivrii <alexi@il.ibm.com>

Co-authored-by: Alexander Ivrii <alexi@il.ibm.com>

This commit deduplicates the code in the built-in routing stage plugins.
First it removes the default plugin which was duplicated with the
stochastic and sabre plugins. There was no functional difference between
just setting the implicit default method name and using a per method
plugin and having a standalone default plugin. Secondly all the vf2 call
limit code is abstracted into a helper function which reduces code
duplication.

Co-authored-by: Luciano Bello <bel@zurich.ibm.com>

Co-authored-by: Luciano Bello <bel@zurich.ibm.com>

The qiskit-toqm project will be one of the first users of this plugin
interface. Once this is released in qiskit, qiskit-toqm will likely
publish their own plugin soon after and if they do we want that plugin
to be used instead of the hardcoded stage in terra. This commit updates
the logic for toqm handling to only use the built-in toqm if a version
of qiskit-toqm is installed without a plugin present.

Co-authored-by: Kevin Hartman <kevin@hart.mn>
Co-authored-by: Toshinari Itoko <15028342+itoko@users.noreply.github.com>

Co-authored-by: Luciano Bello <bel@zurich.ibm.com>