[BUG] Feign OAuth RequestInterceptor not caching access tokens

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

When using the Feign Java client with OAuth2, a new access token is requested for each interaction, even though a valid and not yet expired access token is still in memory. I think the bug is located in this file:

https://github.com/OpenAPITools/openapi-generator/blob/fea42b547ed61dcfdfc479cab6f68a601ec5adde/modules/openapi-generator/src/main/resources/Java/libraries/feign/auth/OAuth.mustache#L80

In line 80 a current time in milliseconds is compared to the expiration time in seconds. This condition is true in (almost) all cases, so a new access token is requested for each API request.

openapi-generator version

I encountered this issue in version 6.0.1 and was able to work around it by downgrading to 5.4.0

OpenAPI declaration file content or url

Generation Details

very similar to https://github.com/OpenAPITools/openapi-generator/blob/master/bin/configs/java-feign-no-nullable.yaml

Steps to reproduce

• Use Feign with OAuth2 (in my case with an OauthClientCredentialsGrant)
• make request to API using generated client -> token is cached in variable accessToken and expiration time in expirationTimeSeconds
• send another request before expirationTimeSeconds is reached -> updateAccessToken() is invoked although the token is not expired

Related issues/PRs

Suggest a fix

change this line:
https://github.com/OpenAPITools/openapi-generator/blob/fea42b547ed61dcfdfc479cab6f68a601ec5adde/modules/openapi-generator/src/main/resources/Java/libraries/feign/auth/OAuth.mustache#L80

to
if (expirationTimeSeconds == null || System.currentTimeMillis() >= expirationTimeSeconds * MILLIS_PER_SECOND) {

Start in modules/openapi-generator/src/main/resources/Java/libraries/feign/auth/OAuth.mustache around line 80 and compare the millisecond time source with expirationTimeSeconds. Reproduce the Feign OAuth client-credentials flow by making two requests before expiration; done means the second request reuses the cached token instead of invoking updateAccessToken().