Add Support for Manta Object Storage as a Gateway (#5025)

Manta is an Object Storage by [Joyent](https://www.joyent.com/)

This PR adds initial support for Manta. It is intended as non-production 
ready so that feedback can be obtained.

vendor/github.com/joyent/triton-go/CHANGELOG.md

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+## Unreleased
+
+## 0.4.1 (December 15)
+
+- Clean up the handling of directory sanitization. Use abs paths everywhere [#79]
+
+## 0.4.0 (December 15)
+
+- Fix an issue where Manta HEAD requests do not return an error resp body [#77]
+- Add support for recursively creating child directories [#78]
+
+## 0.3.0 (December 14)
+
+- Introduce CloudAPI's ListRulesMachines under networking
+- Enable HTTP KeepAlives by default in the client.  15s idle timeout, 2x
+  connections per host, total of 10x connections per client.
+- Expose an optional Headers attribute to clients to allow them to customize
+  HTTP headers when making Object requests.
+- Fix a bug in Directory ListIndex [#69](https://github.com/joyent/issues/69)
+- Inputs to Object inputs have been relaxed to `io.Reader` (formerly a
+  `io.ReadSeeker`) [#73](https://github.com/joyent/issues/73).
+- Add support for ForceDelete of all children of a directory [#71](https://github.com/joyent/issues/71)
+- storage: Introduce `Objects.GetInfo` and `Objects.IsDir` using HEAD requests [#74](https://github.com/joyent/triton-go/issues/74)
+
+## 0.2.1 (November 8)
+
+- Fixing a bug where CreateUser and UpdateUser didn't return the UserID
+
+## 0.2.0 (November 7)
+
+- Introduce CloudAPI's Ping under compute
+- Introduce CloudAPI's RebootMachine under compute instances
+- Introduce CloudAPI's ListUsers, GetUser, CreateUser, UpdateUser and DeleteUser under identity package
+- Introduce CloudAPI's ListMachineSnapshots, GetMachineSnapshot, CreateSnapshot, DeleteMachineSnapshot and StartMachineFromSnapshot under compute package
+- tools: Introduce unit testing and scripts for linting, etc.
+- bug: Fix the `compute.ListMachineRules` endpoint
+
+## 0.1.0 (November 2)
+
+- Initial release of a versioned SDK