Add per-connection driver properties

[ryannedolan]

Summary

• Added connectionProperties parameter to several API methods, including DeployerProvider and ConfigProvider.
• K8sContext now constructs an ApiClient from the connectionProperties, or failing that, from kube/config.
• Dropped defaultContext() and related singleton methods, ensuring the driver is re-entrant and can handle simultaneous connections with different configurations.

Details

Previously, we treated K8sContext as a lazy-loaded singleton, which was shared across connections. This was preventing us from productionizing the driver in a multi-tenant service, where different clients may want to use distinct namespaces, usernames, etc. To solve this problem, we now construct K8sContext dynamically from connection-level properties. This required some minor API changes, in order to thread the connection properties through to the various plugins. In particular, DeployerProvider and ConfigProvider now have access to the connection properties. K8sDeployer etc use this to construct ApiClients etc.

Testing

Verified the driver loads from the CLI, unit tests pass, and integration tests pass.

[jogrogan]

LGTM, we should probably at some point start a wiki to document some of these properties we have defined