out_azure_blob: add support for log_key

[adityasoni99]

Is your feature request related to a problem? Please describe.
By default, the whole log record gets sent to azure_blob.

If you specify a key name with "log_key" option in the azure_blob output plugin configuration, then only the value of that key
should be sent to azure_blob.

Reference PR for out_s3 plugin: #3668

Record without log_key

{"date":"2021-06-16T19:56:28.441428Z","log":"Pi is roughly 3.141605814119434"}

Record with log_key log

Pi is roughly 3.141605814119434