Feature Request: Support route rewrite in http gateway mappings

[maomao94]

Currently, the http gateway in go-zero only supports Path and Method in Mappings, but there is no way to rewrite the route before forwarding to the upstream service.

In many real-world cases, the public API path exposed by the gateway is different from the actual path required by the downstream service. Supporting Rewrite would make the gateway much more flexible.

Example
Upstreams:
  - Name: external-cable
    Http:
      Target: localhost:19103
      Timeout: 15000
    Mappings:
      - Method: POST
        Path: /api/abandoned/external/cable/workList
        Rewrite: /cable/workList

With this configuration:

The gateway exposes:

POST /api/abandoned/external/cable/workList

But it rewrites and forwards the request to:

POST http://localhost:19103/cable/workList

[guonaihong]

这个功能近期会考虑开发。不过要先总体设计下。
计划是yaml的使用伪代码会在这个issue下面讨论下。
先捋一捋大致需要实现的功能点。
比如根据host/qury string/url 各种维度的重写，中间件的引用上。

gateway(http->http)

概念

• match: 用于匹配
• Filters: 做过滤，比如http header，或者host， path, query

filter

新增http header

AddRequestHeader命令，必须接收两个参数，key/value

Upstreams:
  - Name: userservice  # 可选，如果未指定则使用 target
    Http:
      Target: localhost:8080
      Prefix: /api/v1  # 可选
      Timeout: 3000    # 单位为毫秒，默认值 3000
    Mappings:
      - Method: GET
        Path: /users
      - Method: POST
        Path: /users/create
        Filters:
        - AddRequestHeader =X-Request-red, blue

重写http header

TODO

重写path

Upstreams:
  - Name: userservice  # 可选，如果未指定则使用 target
    Http:
      Target: localhost:8080
      Prefix: /api/v1  # 可选
      Timeout: 3000    # 单位为毫秒，默认值 3000
    Mappings:
      - Method: GET
        Path: /users
      - Method: POST
        Path: /users/create
        Filters:
        - RewritePath=/red(?<segment>/?.*), $\{segment}