feat(proxy-rewrite): support multiple same-name headers in headers.add/set

[AlinsRan]

Description

Allow headers.set and headers.add values in proxy-rewrite to be an array, producing multiple headers with the same name:

"proxy-rewrite": {
  "headers": {
    "set": { "X-Api-Version": ["v1", "v2"] }
  }
}

• set with an array replaces any incoming header with the listed values.
• add with an array appends the listed values to the existing ones.
• remove is unchanged (it already takes an array of names).

Why

proxy-rewrite models headers.add/set as a JSON object (map), which structurally cannot express same-name multi-value headers — a JSON object can't hold two values for one key. This is a real gap vs. other gateways that model header transforms as an ordered list of (name, value) pairs (Kong request-transformer append.headers, Envoy request_headers_to_add, HAProxy add-header). The array form closes it. Genuine use cases: gRPC metadata (repeated keys → repeated HTTP/2 headers) and backends/auth proxies that read repeated custom headers.

Fully backward compatible: existing string/number values keep working; the array is an additive schema option. Each element still supports NGINX variables and regex_uri captures.

This also gives the clean answer to the remove + add ordering confusion in #11587 — multi-value is a data-model feature, not an operation-order one.

Which issue(s) this PR fixes

Closes #13163

Changes

• apisix/plugins/proxy-rewrite.lua: schema allows array<string|number> for add/set values; apply loop resolves each element (set → one set_header with a table; add → one add_header per element).
• t/plugin/proxy-rewrite.t: tests for set (replace with multiple) and add (append multiple); t/lib/server.lua: echo helper joining same-name headers.
• docs/{en,zh}/.../proxy-rewrite.md: attribute docs + example.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (if not, please discuss on the APISIX mailing list first)