[google-auth-library-java] Domain Wide Delegation Support: Feature Request List

Given the numerous previous issues regarding domain-wide delegation support in the Java Auth Library, this ticket is to collect them and merge them into a single ticket.

Status updates, additional feature requests, and additional questions can be made here.

For Googlers, see the internal ticket at b/432258797.

# Current support for Domain Wide Delegation
- [x] ServiceAccountCredentials
- [ ] ComputeEngineCredentials
- [ ] ImpersonatedCredentials
- [ ] ... Add additional Credentials that need support

# Changes
Implementation for the `createDelegated()` method from [GoogleCredentials](https://github.com/googleapis/google-auth-library-java/blob/f793c20be7f2f0bd4c112dda45544015d878a083/oauth2_http/java/com/google/auth/oauth2/GoogleCredentials.java#L496)

# Confirmed Existing tickets
1. https://github.com/googleapis/google-cloud-java/issues/12614 (FR for ComputeEngine and Impersonated)
2. https://github.com/googleapis/google-auth-library-java/issues/1774 (FR for Impersonated)
3. https://github.com/googleapis/google-cloud-java/issues/12607 (FR for Impersonated)
4. https://github.com/googleapis/google-cloud-java/issues/12602

## Potential Related tickets
1. https://github.com/googleapis/google-auth-library-java/issues/1270

从 oauth2_http/java/com/google/auth/oauth2/GoogleCredentials.java 及其 createDelegated() 方法开始。查看 ComputeEngineCredentials 和 ImpersonatedCredentials 的已确认工单，然后确定哪些其他凭据类型应纳入范围。完成的标准是：在选定的凭据中实现已达成一致的全域委派支持，并解决功能请求列表。

由索引模型根据 Issue 内容生成。