Support custom options for xtrbackup, xbstream, rclone

[dougfales]

In our usage of the operator, we have found that passing some extra arguments to the xtrabackup, xbstream, and rclone command is sometimes helpful. For instance, if we need to tell these commands to run with higher memory limits or use more threads.

We would like to propose adding a few array properties to the spec that will allow us to pass one or more custom arguments to these commands as they are invoked in the operator.

For example:

• xbstreamAdditionalArguments - extra command line arguments to pass to xbstream
• xtrabackupPrepareAdditionalArguments - extra command line arguments to pass to xtrabackup during --prepare
• xtrabackupAdditionalArguments - extra command line arguments to pass to xtrabackup during --backup
• rcloneAdditionalArguments - extra command line arguments to pass to rclone

I'm hoping to open a PR with these additional options soon.