Use a more resilient s3 object name by default

The s3 plugin uses a default object key that is problematic in a few ways.

1. It makes HEAD requests for each chunk it uploads, starting from 1 each time.
If you have uploaded 2000 log files within the same time slice, it will make
2001 HEAD requests to figure out if it exists.

fluent/fluent-plugin-s3#160

2. The above check is not thread-safe, and two threads can race and decide to
use the same `%{index}` value, with the loser of the race overwriting the chunk
from the winner.

fluent/fluent-plugin-s3#326

This is planned to change for v2, but there's no clear path to v2 right now.
The plugin does warn already if you use multiple threads and don't use either
`%{chunk_id}` or `%{uuid_hash}` in the object key.

pkg/sdk/model/output/s3.go

@@ -127,8 +127,8 @@ type S3OutputConfig struct {
 	GrantFullControl string `json:"grant_full_control,omitempty"`
 	// The length of `%{hex_random}` placeholder(4-16)
 	HexRandomLength string `json:"hex_random_length,omitempty"`
-	// The format of S3 object keys (default: %{path}%{time_slice}_%{index}.%{file_extension})
-	S3ObjectKeyFormat string `json:"s3_object_key_format,omitempty"`
+	// The format of S3 object keys (default: %{path}%{time_slice}_%{uuid_hash}_%{index}.%{file_extension})
+	S3ObjectKeyFormat string `json:"s3_object_key_format,omitempty" plugin:"default:%{path}%{time_slice}_%{uuid_hash}_%{index}.%{file_extension}"`
 	// S3 bucket name
 	S3Bucket string `json:"s3_bucket"`
 	// Archive format on S3

pkg/sdk/model/output/s3_test.go

@@ -39,6 +39,7 @@ buffer:
     @id test
     path logs/${tag}/%Y/%m/%d/
     s3_bucket logging-amazon-s3
+    s3_object_key_format %{path}%{time_slice}_%{uuid_hash}_%{index}.%{file_extension}
     s3_region eu-central-1
     <buffer tag,time>
       @type file

pkg/sdk/model/render/fluent_test.go

@@ -479,6 +479,7 @@ func TestRenderS3(t *testing.T) {
                         @id test
 						path /var/buffer
 						s3_bucket test_bucket
+                                                s3_object_key_format %{path}%{time_slice}_%{uuid_hash}_%{index}.%{file_extension}
 						<buffer tag,time>
 						@type file
 						path asd
@@ -502,6 +503,7 @@ func TestRenderS3(t *testing.T) {
                         @id test
 						path /var/buffer
 						s3_bucket test_bucket
+                                                s3_object_key_format %{path}%{time_slice}_%{uuid_hash}_%{index}.%{file_extension}
 						<instance_profile_credentials>
 						</instance_profile_credentials>`,
 		},
@@ -519,6 +521,7 @@ func TestRenderS3(t *testing.T) {
                         @id test
 						path /var/buffer
 						s3_bucket test_bucket
+                                                s3_object_key_format %{path}%{time_slice}_%{uuid_hash}_%{index}.%{file_extension}
 						<shared_credentials>
 							path e
 							profile_name f