feat: Add support for "title" property

[matteobassan]

Describe the PR
Added support for "title" property, as defined by Openapi Specification Schema Object.
The property is provided via "title" tag on structs.

Relation issue
none

Additional context
The problem arose from having to generate the API in typescript for the client through openapi-generator. Because some fields (marked with "x-nullable") were considered "Inline Models" and the openapi-generator CLI suggested using the "title" field to give it a unique name, i tried to find a ready-made solution with no luck.

The "schema" struct already contained the "Title" field, but this is never assigned, so I proceeded to complete the assignment of the field through the "title" tag.

Usage:

the following definition:

type CreatePageRequest struct {
	Title       string                `json:"title" validate:"required,max=255"`
	CoverImage  *EmbeddedImageRequest `json:"coverImage,omitempty" extensions:"x-nullable" title:"EmbeddedImageRequest"`
}

will generate this:

...
{
"title": {
                    "type": "string",
                    "maxLength": 255
                },
"coverImage": {
                    "format": "url",
                    "title": "EmbeddedImageRequest",
                    "allOf": [
                        {
                            "$ref": "#/definitions/EmbeddedImageRequestInternal"
                        }
                    ],
                    "x-nullable": true
                },
}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (56fde5c) 83.35% compared to head (a86b4e3) 83.36%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1762   +/-   ##
=======================================
  Coverage   83.35%   83.36%           
=======================================
  Files          16       16           
  Lines        3870     3872    +2     
=======================================
+ Hits         3226     3228    +2     
  Misses        545      545           
  Partials       99       99           

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