RFC-13: Add Gateway Pair (#13)

* RFC-1: Add a pair for Reacher

* Add more details

docs/rfcs/0-example.md

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+- Author: (fill me in with `name <mail>`, e.g., zu1k <[email protected]>)
+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- RFC PR: [beyondstorage/go-service-ipfs#0](https://github.com/beyondstorage/go-service-ipfs/issues/0)
+- Tracking Issue: [beyondstorage/go-service-ipfs#0](https://github.com/beyondstorage/go-service-ipfs/issues/0)
+
+# RFC-0: <proposal name>
+
+- Updates: (delete this part if not applicable)
+    - [GSP-20](./20-abc): Deletes something
+- Updated By: (delete this part if not applicable)
+    - [GSP-10](./10-do-be-do-be-do): Adds something
+    - [GSP-1000](./1000-lalala): Deprecates this RFC
+
+## Background
+
+Explain why we are doing this.
+
+Related issues and early discussions can be linked, but the RFC should try to be self-contained if possible.
+
+## Proposal
+
+<proposal's content>
+
+## Rationale
+
+<proposal's rationale content, other implementations>
+
+Possible content:
+
+- Design Principles
+- Drawbacks
+- Alternative implementations and comparison
+- Possible Q&As
+
+## Compatibility
+
+<proposal's compatibility statement>
+
+## Implementation
+
+Explain what steps should be done to implement this proposal.

docs/rfcs/13-add-gateway-pair.md

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+- Author: zu1k <[email protected]>
+- Start Date: 2021-07-15
+- RFC PR: [beyondstorage/go-service-ipfs#13](https://github.com/beyondstorage/go-service-ipfs/pull/13)
+- Tracking Issue: [beyondstorage/go-service-ipfs#14](https://github.com/beyondstorage/go-service-ipfs/issues/14)
+
+# RFC-13: Add Gateway Pair
+
+Releated issue: [beyondstorage/go-service-ipfs#5](https://github.com/beyondstorage/go-service-ipfs/issues/5)
+
+## Background
+
+With IPFS, we can get the `CID` of an object, then we can use it with a public or internal IPFS Gateway to splice out the access link.
+
+When using internal gateway like `127.0.0.1:8080`, the access link are inaccessible from external network.
+
+When using a public gateway, e.g. `ipfs.io`, `cf-ipfs.com`, if the service is an intranet IPFS cluster, it is possible that the files are not accessible from these public gateway as well.
+
+### How to get the `CID` of an object?
+
+We can [stat](https://docs.ipfs.io/reference/http/api/#api-v0-files-stat) the object, the `hash` would be its `CID`.
+
+```
+$ ipfs files stat /part1
+QmSvxBhBHn2gu9kAdjCTYfUgDNnyBcNoXXVojRyKdBjZUA  # CID
+Size: 262144
+CumulativeSize: 262158
+ChildBlocks: 0
+Type: file
+```
+
+### How to splice out the access link?
+
+- https://{gateway URL}/ipfs/{CID}/{optional path to resource}
+- https://{CID}.ipfs.{gatewayURL}/{optional path to resource}
+
+## Proposal
+
+I propose to add a pair to let the user specify the `gateway`.
+
+- The `type` of `gateway` should be `String`
+- The `format` of `gateway` should follow [go-endpoint](https://github.com/beyondstorage/go-endpoint/blob/master/README.md)
+- The `value` of `gateway` should be parsed into `HTTP` or `HTTPS`
+- Now we use `gateway` only in `Reach` operation
+
+## Rationale
+
+### Why not infer the `gateway` from the `endpoint`?
+
+If the `endpoint` used is not the loopback address, it is possible to infer the `gateway` address.
+
+For example, if a user tells IPFS to listen to `0.0.0.0` and use the public IP to access, the `endpoint` may be `<IP>:5001` or `<domain>:5001`, we can infer the `gateway` as `<IP>:8080` or `<domain>:8080`.
+
+The problem with this approach is that it is very unstable and the IPFS api does not come with a forensic mechanism, so it is almost impossible for users to access directly through the public IP, so the inferred gateway is most likely invalid.
+
+### Why not use the `gateway` in the configuration?
+
+We can get the `gateway` of IPFS through the [config-show api](https://docs.ipfs.io/reference/http/api/#api-v0-config-show).
+
+This method works if the user has modified the default gateway. 
+
+However, we cannot guarantee that the user has modified this configuration item, and it may be more common for users to use reverse proxy to make IPFS public.
+
+## Compatibility
+
+No compatibility issues at this time.
+
+## Implementation
+
+First we define a pair of `String` type with the name `gateway` in `service.toml`. Then we `generate` code and implement the `Reacher` interface.