Implement {Put,Get,Delete}BucketCors and CORS in general

- OPTIONS request against API endpoint
- Returning corresponding CORS headers on API calls
- Returning corresponding CORS headers on website GET's

doc/book/src/reference_manual/s3_compatibility.md

@@ -9,7 +9,8 @@ Implemented:
 - putting and getting objects in buckets
 - multipart uploads
 - listing objects
-- access control on a per-key-per-bucket basis
+- access control on a per-access-key-per-bucket basis
+- CORS headers on web endpoint
 
 Not implemented:
 
@@ -31,9 +32,11 @@ All APIs that are not mentionned are not implemented and will return a 501 Not I
 | CreateBucket                 | Implemented                      |
 | CreateMultipartUpload        | Implemented                      |
 | DeleteBucket                 | Implemented                      |
+| DeleteBucketCors             | Implemented                      |
 | DeleteBucketWebsite          | Implemented                      |
 | DeleteObject                 | Implemented                      |
 | DeleteObjects                | Implemented                      |
+| GetBucketCors                | Implemented                      |
 | GetBucketLocation            | Implemented                      |
 | GetBucketVersioning          | Stub (see below)                 |
 | GetBucketWebsite             | Implemented                      |
@@ -46,6 +49,7 @@ All APIs that are not mentionned are not implemented and will return a 501 Not I
 | ListMultipartUpload          | Implemented                      |
 | ListParts                    | Implemented                      |
 | PutObject                    | Implemented                      |
+| PutBucketCors                | Implemented                      |
 | PutBucketWebsite             | Partially implemented (see below)|
 | UploadPart                   | Implemented                      |
 | UploadPartCopy               | Implemented                      |

doc/book/src/working_documents/compatibility_target.md

@@ -26,10 +26,10 @@ your motivations for doing so in the PR message.
 | 							 | UploadPart |
 | 							 | [*ListMultipartUploads*](https://git.deuxfleurs.fr/Deuxfleurs/garage/issues/103) |
 | 							 | [*ListParts*](https://git.deuxfleurs.fr/Deuxfleurs/garage/issues/103) |
-| **A-tier** (will implement)     | |
-| 							 | [*GetBucketCors*](https://git.deuxfleurs.fr/Deuxfleurs/garage/issues/138) |
-| 							 | [*PutBucketCors*](https://git.deuxfleurs.fr/Deuxfleurs/garage/issues/138) |
-| 							 | [*DeleteBucketCors*](https://git.deuxfleurs.fr/Deuxfleurs/garage/issues/138) |
+| **A-tier**                 | |
+| 							 | GetBucketCors |
+| 							 | PutBucketCors |
+| 							 | DeleteBucketCors |
 | 							 | UploadPartCopy |
 | 							 | GetBucketWebsite |
 | 							 | PutBucketWebsite |