Add content unit option to customize referencable entities

[vearutop]

This allows setting up request/response references with ease, also it allows for direct access to respective entities for fine-tuning (setting uncommon properties).

	r := openapi31.NewReflector()

	op, err := r.NewOperationContext(http.MethodPost, "/{document_id}/{client}")
	require.NoError(t, err)

	op.AddReqStructure(new(struct {
		DocumentID string `path:"document_id"`
		Client     string `path:"client"`
		Foo        int    `json:"foo"`
	}), openapi.WithCustomize(func(cor openapi.ContentOrReference) {
		_, ok := cor.(*openapi31.RequestBodyOrReference)
		assert.True(t, ok)

		cor.SetReference("../somewhere/components/requests/foo.yaml")
	}))

	op.AddRespStructure(
		nil, openapi.WithReference("../somewhere/components/responses/204.yaml"), openapi.WithHTTPStatus(204),
	)
	op.AddRespStructure(
		nil, openapi.WithCustomize(func(cor openapi.ContentOrReference) {
			_, ok := cor.(*openapi31.ResponseOrReference)
			assert.True(t, ok)

			cor.SetReference("../somewhere/components/responses/200.yaml")
		}), openapi.WithHTTPStatus(200),
	)

	require.NoError(t, r.AddOperation(op))

	assertjson.EqMarshal(t, `{
	  "openapi":"3.1.0","info":{"title":"","version":""},
	  "paths":{
		"/{document_id}/{client}":{
		  "post":{
			"parameters":[
			  {
				"name":"document_id","in":"path","required":true,
				"schema":{"type":"string"}
			  },
			  {
				"name":"client","in":"path","required":true,
				"schema":{"type":"string"}
			  }
			],
			"requestBody":{"$ref":"../somewhere/components/requests/foo.yaml"},
			"responses":{
			  "200":{"$ref":"../somewhere/components/responses/200.yaml"},
			  "204":{"$ref":"../somewhere/components/responses/204.yaml"}
			}
		  }
		}
	  }
	}`, r.SpecSchema())

[github-actions]

Lines Of Code

Language	Files	Lines	Code	Comments	Blanks	Complexity	Bytes
Go	21	4032 (+74)	2991 (+46)	227 (+13)	814 (+15)	819 (+12)	92.7K (+1.7K)
Go (test)	22	6545 (+114)	5020 (+98)	660	865 (+16)	61	168.5K (+3.1K)

[github-actions]

Go API Changes

# github.com/swaggest/openapi-go
## compatible changes
ContentOrReference: added
ContentUnit.Customize: added
WithCustomize: added
WithReference: added

# github.com/swaggest/openapi-go/openapi3
## compatible changes
(*RequestBodyOrRef).SetReference: added
(*ResponseOrRef).SetReference: added

# github.com/swaggest/openapi-go/openapi31
## compatible changes
(*RequestBodyOrReference).SetReference: added
(*ResponseOrReference).SetReference: added

# summary
Inferred base version: v0.2.53
Suggested version: v0.3.0

[github-actions]

Unit Test Coverage

total: (statements) 47.4%
changed lines: (statements) 81.5%, coverage is less than 90.0%, consider testing the changes more thoroughly

Coverage of changed lines

File	Function	Coverage
Total		81.5%
openapi3/helper.go		100.0%
openapi3/helper.go:215	SetReference	100.0%
openapi3/helper.go:221	SetReference	100.0%
openapi3/reflect.go		100.0%
openapi3/reflect.go:271	setupRequest	100.0%
openapi3/reflect.go:620	setupResponse	100.0%
openapi31/helper.go		100.0%
openapi31/helper.go:274	SetReference	100.0%
openapi31/helper.go:280	SetReference	100.0%
openapi31/reflect.go		100.0%
openapi31/reflect.go:218	setupRequest	100.0%
openapi31/reflect.go:571	setupResponse	100.0%
operation.go		0.0%
operation.go:59	WithCustomize	0.0%
operation.go:66	WithReference	0.0%

Coverage diff with base branch

File	Function	Base Coverage	Current Coverage
Total		47.1%	47.4% (+0.3%)
openapi3/entities.go	RequestBodyReferenceEns	0.0%	100.0% (+100.0%)
openapi3/entities.go	ResponseReferenceEns	0.0%	100.0% (+100.0%)
openapi3/helper.go	SetReference	no function	100.0%
openapi3/reflect.go	setupRequest	58.3%	64.3% (+6.0%)
openapi3/reflect.go	setupResponse	83.3%	84.8% (+1.5%)
openapi31/helper.go	SetReference	no function	100.0%
openapi31/reflect.go	setupRequest	66.7%	71.4% (+4.7%)
openapi31/reflect.go	setupResponse	83.3%	84.8% (+1.5%)
operation.go	WithCustomize	no function	0.0%
operation.go	WithReference	no function	0.0%