Make issued_token_type optional to support OAuth2 Client Credential Flow

[flyrain]

To fix issue(#463). In short, Client Credential Flow's http response doesn't have the field issued_token_type. Make it optional to avoid validation error like this:

ValidationError: 1 validation error for TokenResponse
issued_token_type
  Field required [type=missing, input_value={'access_token': 'eyJhbGc... 'token_type': 'bearer'},
input_type=dict]

cc @danielcweeks @Fokko @syun64 @RussellSpitzer

[sungwy]

Hey @flyrain thank you very much for putting this together! I glossed over this discrepancy because PyIceberg was in 'developing' stages. It's exciting how fast this project is maturing!

I left a comment to point out a few other discrepancies in the existing TokenResponse model that I thought would also be great to fix this time around. Let me know what you think!

[sungwy]

Could we use this opportunity to align the OAuthTokenResponse exactly with the model specified in the Iceberg Rest Catalog Open API Spec?

• we want to verify within the model that the issued_token_type is one of accepted types, if it is provided
• the current model is missing Optional scope and refresh_token
• also update expires_in to be Optional (expires_in is a recommended property in OAuth and if it wasn't provided in the TokenResponse, it will fail for similar reasons in the existing model)

[flyrain]

That's good ideas. Thanks for the review, @syun64. Fixed in a new commit.

[sungwy]

Looks good from my side. Let's seek approval from @Fokko @danielcweeks to get this merged in 👍

[flyrain]

Thanks @syun64 for the review. We will need at least an approval from committers. cc @Fokko @danielcweeks

[Fokko]

Thank you @flyrain for fixing this 👍 I wasn't aware of this discrepancy!

Thanks @syun64 for the great review 🙌

[Fokko]

We could also convert this into a literal, since it is an enum in the spec:

Suggested change

	issued_token_type: Optional[str] = Field(default=None)
	issued_token_type: Optional[Literal['urn:ietf:params:oauth:token-type:access_token', 'urn:ietf:params:oauth:token-type:refresh_token', 'urn:ietf:params:oauth:token-type:id_token', 'urn:ietf:params:oauth:token-type:saml1', 'urn:ietf:params:oauth:token-type:saml2', 'urn:ietf:params:oauth:token-type:jwt'] = Field(default=None)

[flyrain]

Thanks @Fokko for the review!