Rework CustomJWTAuthentication to request the oauth token correctly

[nclaeys]

Usecase:
I am using oidc on k8s with workload-identity for spark applications and noticed that requesting a token did not work.

Problem:
The fix for #437 is not sufficient to work with workload identity as the new CustomJwtAuthentication class does not construct the correct body for requesting a token (the applyTo method does nothing).

I use this library as follows:

IClientCredential credential = ClientCredentialFactory.createFromClientAssertion(tokenFileString);
ConfidentialClientApplication app = ConfidentialClientApplication.builder(clientId, credential).authority(authority).build();

Set<String> scopes = new HashSet<>();
scopes.add("https://storage.azure.com/.default");

ClientCredentialParameters parameters = ClientCredentialParameters.builder(scopes).tenant(tenantId).build();
IAuthenticationResult token = app.acquireToken(parameters).get(5, TimeUnit.SECONDS);

Changes that I made are:

• I updated the customJwtAuthentication to work similarly to how the JwtAuthentication class from com.nimbusds.oauth2.sdk.auth. I think this is useful for other users as well, such that they can also use workload identity with java applications.
• The clientId used, should be the azure ad application id instead of the subject in the jwt token as this is the serviceAccount and thus does not exist in azure ad.

Note on running all tests:
I had issues running all tests as some depended on libraries I do not have on Linux. To test the relevant classes I created a custom keystore with a self signed certificate in it with a password (minimum was 6 characters). Can you describe the setup required to run all the tests locally?

[siddhijain]

@nclaeys Thanks for putting your time and effort in fixing the code. We highly appreciate it. I have tested the changes and they look good to me. I am not sure of the setup needed to run these tests in Linux since we use a Windows machine. I will update you once I find something.
I will discuss with my team and wait for them to review the PR before merging this in.
Thanks again.