Feature: Load custom Config Sourcer plugins

[briancain]

Is your feature request related to a problem? Please describe.

As a Waypoint developer, there's no way for me to add a custom config sourcer plugin without re-building the whole world and deploy my own Waypoint. Currently, config sourcer plugins are hard coded:

waypoint/internal/plugin/plugin.go

Lines 88 to 113 in ed4b99d

	// ConfigSourcers are the list of built-in config sourcers. These will
	// eventually be moved out to exec-based plugins but for now we just
	// hardcode them. This is used by the CEB.
	ConfigSourcers = map[string]*Instance{
	"aws-ssm": {
	Component: &ssm.ConfigSourcer{},
	},
	"kubernetes": {
	Component: &k8s.ConfigSourcer{},
	},
	"null": {
	Component: &null.ConfigSourcer{},
	},
	"vault": {
	Component: &vault.ConfigSourcer{},
	},
	"terraform-cloud": {
	Component: &tfc.ConfigSourcer{},
	},
	"consul": {
	Component: &consul.ConfigSourcer{},
	},
	"packer": {
	Component: &packer.ConfigSourcer{},
	},
	}

This means, the only way I can get Waypoint to load my plugin is to build my own Waypoint.

Describe the solution you'd like

Load config sourcer plugins in the same way we load our build, deploy, release plugins so that users can bring their own config sourcers.

Describe alternatives you've considered

Nothing.

Explain any additional use-cases

Additional context