Support new fields for /v1/images/generation API

[mikecutalo]

Describe the change
Updating the /v1/images/generation api with the new changes announced at devday.

Describe your solution
Simply add new fields to ImageRequest & ImageResponseDataInner as well a bunch of new const's for supported options.

Tests
Added more fields to the existing TestImages test, as these are later unmarshaled. Although if you would like to see tests that simply assert marshalling I could add some, let me know.

Issue: #529

[vuguet]

could you set the response on this way ? :

	type ImageResponseDataInner struct {
		URL     string `json:"url,omitempty"`
		B64JSON string `json:"b64_json,omitempty"`
		RevisedPrompt string `json:"revised_prompt"`
	}

They now return a prompt in the response :

[vuguet]

It is not related to your changes and sorry for so many comments but the linter issue is because of this :

[mikecutalo]

Nice good catch let me update the response object. There is already a lint fix PR #522 thats been opened.

[codecov]

Codecov Report

Merging #530 (924bd06) into master (3063e67) will not change coverage.
Report is 2 commits behind head on master.
The diff coverage is n/a.

❗ Current head 924bd06 differs from pull request most recent head bbfebb4. Consider uploading reports for the commit bbfebb4 to get more accurate results

@@           Coverage Diff           @@
##           master     #530   +/-   ##
=======================================
  Coverage   97.85%   97.85%           
=======================================
  Files          20       20           
  Lines         978      978           
=======================================
  Hits          957      957           
  Misses         15       15           
  Partials        6        6           

Files	Coverage Δ
chat.go	100.00% <ø> (ø)
chat_stream.go	100.00% <ø> (ø)
image.go	92.59% <ø> (ø)

[vvatanabe]

@mikecutalo
Sometimes you may want to explicitly specify default values in the code. Please add the following as well.

CreateImageQualityStandard = "standard"

https://platform.openai.com/docs/api-reference/images/create#images-create-quality

[vvatanabe]

@mikecutalo Thanks for implementing the new API so quickly!