[REQ] Pattern validation in Go generator

Is your feature request related to a problem? Please describe.

The go generator currently doesn't support any form of string validation.

OpenAPI has a pattern parameter on properties that allows specifying a validation regex, and this is supported for some generators, but not Go.

Describe the solution you'd like

There are several Go packages available that support regex validation on struct fields via annotations.

I'm using https://github.com/go-validator/validator/tree/v2 at the moment, with structs of this form:

type MyObject struct {
	// An IPv4 address
	IPAddress string `json:"IPAddress" validate:"regexp=^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$"`
}

This struct should be generated by an OpenAPI definition like this:

MyObject:
  type: object
  properties:
    IPAddress:
      type: string
      description: An IPv4 address
      pattern: '^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$'

Describe alternatives you've considered

Since the pattern field doesn't do anything, I'm using x-go-custom-tag instead, but this isn't portable to other languages:

MyObject:
  type: object
  properties:
    IPAddress:
      type: string
      description: An IPv4 address
      x-go-custom-tag: validate:"regexp=^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}$"

Additional context

Start with the Go generator's handling of OpenAPI string properties and the pattern parameter, then review how x-go-custom-tag is currently applied. Done means an OpenAPI pattern produces the requested validation annotation in the generated Go struct, with behavior matching the IPv4 example.