First version of admin API (#298)

**Spec:**

- [x] Start writing
- [x] Specify all layout endpoints
- [x] Specify all endpoints for operations on keys
- [x] Specify all endpoints for operations on key/bucket permissions
- [x] Specify all endpoints for operations on buckets
- [x] Specify all endpoints for operations on bucket aliases

View rendered spec at <https://git.deuxfleurs.fr/Deuxfleurs/garage/src/branch/admin-api/doc/drafts/admin-api.md>

**Code:**

- [x] Refactor code for admin api to use common api code that was created for K2V

**General endpoints:**

- [x] Metrics
- [x] GetClusterStatus
- [x] ConnectClusterNodes
- [x] GetClusterLayout
- [x] UpdateClusterLayout
- [x] ApplyClusterLayout
- [x] RevertClusterLayout

**Key-related endpoints:**

- [x] ListKeys
- [x] CreateKey
- [x] ImportKey
- [x] GetKeyInfo
- [x] UpdateKey
- [x] DeleteKey

**Bucket-related endpoints:**

- [x] ListBuckets
- [x] CreateBucket
- [x] GetBucketInfo
- [x] DeleteBucket
- [x] PutBucketWebsite
- [x] DeleteBucketWebsite

**Operations on key/bucket permissions:**

- [x] BucketAllowKey
- [x] BucketDenyKey

**Operations on bucket aliases:**

- [x] GlobalAliasBucket
- [x] GlobalUnaliasBucket
- [x] LocalAliasBucket
- [x] LocalUnaliasBucket

**And also:**

- [x] Separate error type for the admin API (this PR includes a quite big refactoring of error handling)
- [x] Add management of website access
- [ ] Check that nothing is missing wrt what can be done using the CLI
- [ ] Improve formatting of the spec
- [x] Make sure everyone is cool with the API design

Fix #231
Fix #295

Co-authored-by: Alex Auvolat <alex@adnab.me>
Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/298
Co-authored-by: Alex <alex@adnab.me>
Co-committed-by: Alex <alex@adnab.me>

src/api/admin/error.rs

@@ -0,0 +1,96 @@
+use err_derive::Error;
+use hyper::header::HeaderValue;
+use hyper::{Body, HeaderMap, StatusCode};
+
+pub use garage_model::helper::error::Error as HelperError;
+
+use crate::common_error::CommonError;
+pub use crate::common_error::{CommonErrorDerivative, OkOrBadRequest, OkOrInternalError};
+use crate::generic_server::ApiError;
+use crate::helpers::CustomApiErrorBody;
+
+/// Errors of this crate
+#[derive(Debug, Error)]
+pub enum Error {
+	#[error(display = "{}", _0)]
+	/// Error from common error
+	Common(CommonError),
+
+	// Category: cannot process
+	/// The API access key does not exist
+	#[error(display = "Access key not found: {}", _0)]
+	NoSuchAccessKey(String),
+
+	/// In Import key, the key already exists
+	#[error(
+		display = "Key {} already exists in data store. Even if it is deleted, we can't let you create a new key with the same ID. Sorry.",
+		_0
+	)]
+	KeyAlreadyExists(String),
+}
+
+impl<T> From<T> for Error
+where
+	CommonError: From<T>,
+{
+	fn from(err: T) -> Self {
+		Error::Common(CommonError::from(err))
+	}
+}
+
+impl CommonErrorDerivative for Error {}
+
+impl From<HelperError> for Error {
+	fn from(err: HelperError) -> Self {
+		match err {
+			HelperError::Internal(i) => Self::Common(CommonError::InternalError(i)),
+			HelperError::BadRequest(b) => Self::Common(CommonError::BadRequest(b)),
+			HelperError::InvalidBucketName(n) => Self::Common(CommonError::InvalidBucketName(n)),
+			HelperError::NoSuchBucket(n) => Self::Common(CommonError::NoSuchBucket(n)),
+			HelperError::NoSuchAccessKey(n) => Self::NoSuchAccessKey(n),
+		}
+	}
+}
+
+impl Error {
+	fn code(&self) -> &'static str {
+		match self {
+			Error::Common(c) => c.aws_code(),
+			Error::NoSuchAccessKey(_) => "NoSuchAccessKey",
+			Error::KeyAlreadyExists(_) => "KeyAlreadyExists",
+		}
+	}
+}
+
+impl ApiError for Error {
+	/// Get the HTTP status code that best represents the meaning of the error for the client
+	fn http_status_code(&self) -> StatusCode {
+		match self {
+			Error::Common(c) => c.http_status_code(),
+			Error::NoSuchAccessKey(_) => StatusCode::NOT_FOUND,
+			Error::KeyAlreadyExists(_) => StatusCode::CONFLICT,
+		}
+	}
+
+	fn add_http_headers(&self, _header_map: &mut HeaderMap<HeaderValue>) {
+		// nothing
+	}
+
+	fn http_body(&self, garage_region: &str, path: &str) -> Body {
+		let error = CustomApiErrorBody {
+			code: self.code().to_string(),
+			message: format!("{}", self),
+			path: path.to_string(),
+			region: garage_region.to_string(),
+		};
+		Body::from(serde_json::to_string_pretty(&error).unwrap_or_else(|_| {
+			r#"
+{
+	"code": "InternalError",
+	"message": "JSON encoding of error failed"
+}
+			"#
+			.into()
+		}))
+	}
+}