adjusted negotiation logic so that it is possible to add or overwrite…

[kkoehler]

… render logic for sepcific content-types and added/referenced a default protobuf renderer.

The following code is a sample which shows the handling for ProtoBuf data. The old API is still working.

package main

import (
	"github.com/gin-gonic/gin"
	"github.com/gin-gonic/gin/binding"
	"google.golang.org/protobuf/types/known/structpb"
)

func main() {
	g := gin.Default()

	testData, _ := structpb.NewStruct(map[string]interface{}{
		"key": "value",
	})

	negotiate := gin.NewNegotiate([]string{
		binding.MIMEJSON,
		binding.MIMEPROTOBUF,
	})
	g.GET("/negotiate", func(c *gin.Context) {
		c.Negotiate(200, negotiate.WithData(testData))
	})

	g.Run(":8080")
}

[kkoehler]

resolves: #3799

[appleboy]

@kkoehler Please add more testing.

[Copilot]

Pull Request Overview

This PR refactors the content negotiation logic to use a pluggable map of render functions, enables adding or overriding renderers, and introduces a default Protobuf renderer.

• Introduce NewNegotiate and fluent WithData for building negotiation configs
• Replace the hard-coded switch in Context.Negotiate with a lookup on negotiationRenderMappings
• Add AddNegotiationRenderMapping API and include MIMEPROTOBUF in the default mappings

Comments suppressed due to low confidence (1)

context.go:1278

• [nitpick] The parameter name binding shadows the imported binding package and may be confusing. Consider renaming it to renderFunc or fn for clarity.

func AddNegotiationRenderMapping(mimeType string, binding NegotiationRenderFunc) {

[kkoehler]

@kkoehler Please add more testing.

@appleboy added some tests to the pull request