extproc: implement ClientFilter and ClientFilterBuilder interface (#9086)

This PR is part of implementing [A93:
xds-ext-proc](https://github.com/grpc/proposal/blob/master/A93-xds-ext-proc.md)

This PR adds the builder that implements the
[ClientFilter](https://github.com/grpc/grpc-go/blob/ac4aa01bd485f3a574469d7985cf55e79abf44a9/internal/xds/httpfilter/httpfilter.go#L80)
and
[ClientFilterBuilder](https://github.com/grpc/grpc-go/blob/ac4aa01bd485f3a574469d7985cf55e79abf44a9/internal/xds/httpfilter/httpfilter.go#L71)
interface. That includes creating the interceptor config from the base
and the override config. It also includes making the ext_proc channel.

THis PR has a placeholder function for converting grpcService to channel
The function will be implemented later when implementing [gRFC
A102](https://github.com/grpc/proposal/A102-xds-grpc-service.md).

The `interceptor` struct has a `resolver.ClientInterceptor` embedded for
now, but that will be removed in a later PR when we implement the
`resolver.ClientInterceptor` interface. Since the filter will not be
registered and no one is calling the `resolver.ClientInterceptor`
function fro extproc filter , it will not panic.

#ext-proc-a93

RELEASE NOTES: None

Author: eshitachandwani

internal/xds/httpfilter/extproc/config.go

@@ -156,3 +156,28 @@ func processingModesFromProto(pm *v3procfilterpb.ProcessingMode) processingModes
 		responseTrailerMode: resolveHeaderMode(pm.GetResponseTrailerMode(), modeSkip),
 	}
 }
+
+// newInterceptorConfig creates the interceptor config from the base and
+// override filter configs. If a field is set in both the base and override
+// configs, the value from the override config will be used.
+func newInterceptorConfig(base baseConfig, override overrideConfig) baseConfig {
+	ic := base
+
+	// Apply overrides if present.
+	if val, ok := override.server.Value(); ok {
+		ic.server = val
+	}
+	if val, ok := override.failureModeAllow.Value(); ok {
+		ic.failureModeAllow = val
+	}
+	if override.requestAttributes != nil {
+		ic.requestAttributes = override.requestAttributes
+	}
+	if override.responseAttributes != nil {
+		ic.responseAttributes = override.responseAttributes
+	}
+	if val, ok := override.processingModes.Value(); ok {
+		ic.processingModes = val
+	}
+	return ic
+}