Checking error strings makes these tests flaky, especially if the
error string is changed in libcni. Have gone ahead an introduced a new
error type `NoConfigsFoundError` and the Match is against the error
type making it more deterministic.
Add a new CapabilityArgs member to the RuntimeConf struct which runtimes can
use to pass arbitrary capability-based keys to the plugin. Elements of this
member will be filtered against the plugin's advertised capabilities (from
its config JSON) and then added to a new "runtimeConfig" top-level map added
to the config JSON sent to the plugin on stdin.
Also "runtime_config"->"runtimeConfig" in CONVENTIONS.md to make
capitalization consistent with other CNI config keys like "cniVersion".
Using a new ".configlist" file format that allows specifying
a list of CNI network configurations to run, add new libcni
helper functions to call each plugin in the list, injecting
the overall name, CNI version, and previous plugin's Result
structure into the configuration of the next plugin.
