Agent auto auth wrapping new config checks (#6479)

* Simplify Run(): the function that was being sent over a channel doesn't
need to close over anything except latestToken, and we don't need to
create a new one each iteration.  Instead just pass the relevant items,
namely the token and sink to work on.

* Disallow the following config combinations:
1. auto_auth.method.wrap_ttl > 0 and multiple file sinks
2. auto_auth.method.wrap_ttl > 0 and single file sink with wrap_ttl > 0
3. auto_auth.method.wrap_ttl > 0 and cache.use_auto_auth_token = true

* Expose errors that occur when APIProxy is forwarding request to Vault.

* Fix merge issues.

command/agent/cache/api_proxy.go

@@ -45,6 +45,9 @@ func (ap *APIProxy) Send(ctx context.Context, req *SendRequest) (*SendResponse,
 	ap.logger.Info("forwarding request", "path", req.Request.URL.Path, "method", req.Request.Method)
 
 	resp, err := client.RawRequestWithContext(ctx, fwReq)
+	if resp == nil && err != nil {
+		return nil, err
+	}
 
 	// Before error checking from the request call, we'd want to initialize a SendResponse to
 	// potentially return