Add JSON key content for auth

[omkar-g]

Release notes

Added optional field json_key_file_content to get Service Account JSON key as an String input.

What does this PR do?

Add optional field json_key_file_content to get Service Account JSON key as an String input for authentication.

This will add one more option for user to pass Service Account JSON key. With this option the value of Service Account JSON key can be passed as String, instead of file path.

Why is it important/What is the impact to the user?

This change will not force user to deal with file upload for Service Account JSON key and it can be given as String.

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have made corresponding change to the default configuration files (and/or docker env variables)
• [x] I have added tests that prove my fix is effective or that my feature works

How to test this PR locally

Use the field json_key_file_content field instead of json_key_file in .conf file of logstash. And run the application outside of GCE.

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