support access log aliases for text-based logging

[skriss]

When using JSON access logging, a set of Contour-specific aliases are supported that are more user-friendly and get translated into underlying Envoy configuration:

contour/apis/projectcontour/v1alpha1/accesslog.go

Lines 52 to 66 in b5acf7f

	// jsonFields is the canonical translation table for JSON fields to Envoy log template formats,
	// used for specifying fields for Envoy to log when JSON logging is enabled.
	var jsonFields = map[string]string{
	"@timestamp": "%START_TIME%",
	"ts": "%START_TIME%",
	"authority": "%REQ(:AUTHORITY)%",
	"method": "%REQ(:METHOD)%",
	"path": "%REQ(X-ENVOY-ORIGINAL-PATH?:PATH)%",
	"request_id": "%REQ(X-REQUEST-ID)%",
	"uber_trace_id": "%REQ(UBER-TRACE-ID)%",
	"upstream_service_time": "%RESP(X-ENVOY-UPSTREAM-SERVICE-TIME)%",
	"user_agent": "%REQ(USER-AGENT)%",
	"x_forwarded_for": "%REQ(X-FORWARDED-FOR)%",
	"x_trace_id": "%REQ(X-TRACE-ID)%",
	}

These are currently only supported for JSON access logging, but I think it'd be reasonable to support them for text-based access logging as well.

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This bot triages Issues according to the following rules:

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