fix: check bucket owner permission

[saikiranakula-amzn]

Issue #, if available:

Description of changes:
Check bucket owner permission before retrieving credentialspec
Testing done:

[ec2-user@EC2AMAZ-Ll40tS tests]$ sudo ./gmsa_test_client --create_kerberos_tickets_arn arn:aws:s3:::gmsacredspec/gmsa-cred-spec.json#3456/test xxxx xxxx xxxx us-west-2
krb tickets will get created
created ticket for gMSA /var/credentials-fetcher/krbdir/3456/test
Client received output for add kerberos arn lease: 3456
provide a valid arg, for help use -h or --help

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[jamolina]

Looks good