I have created a service account on Google Cloud Platform. I am using
a standalone Java program which uses GCP Java Client API to get the
Authentication Token ID by taking service account JSON file. By using
the Token ID, I can create a compute instance.
Using a service account is the correct and recommended method to authenticate and authorize software applications.
Note. The assumption here is that the software application is running on your systems under your control. If instead you are installing software on a user's desktop or system not under your control, then you would use Google OAuth 2.0 (Google Accounts) to obtain User Credentials to authorize your application.
Another method is to issue short-lived temporary credentials from a service account credential that are time limited. These short-lived credentials are created on your server and then handed to the client.
I don't want to use Java API or gcloud tool, is there a REST API
exists which takes required details and return Token ID?
You have the classic "Chicken or Egg" situation. You need credentials to authenticate and authorize otherwise anyone could create credentials. Google Service Account credentials provides this. To create service account credentials, use the Google Cloud Console or gcloud CLI to download the service account Json file.
I went through the GCP documentation and could not find any details
for the authentication and authorization through REST using Service
Account.
This question is confusing. Do you want to use Service Account credentials to authorize your Google API calls OR do you want to call a Google API to obtain credentials?
In the first case, once you create service account credentials, they are used to authorize your API calls. You add the Access Token to the HTTP header when making API calls.
For the second case, use Google OAuth 2.0 to obtain credentials. Google OAuth 2.0 uses Google Accounts for authentication. This method provides you with an Access Token (just like a service account) and a Refresh Token and Client ID token. You will need to add the Google Accounts user identity to your Google Cloud IAM which provides for authorization (privileges).
