Feature: Adds build variables to deployBranchCmd

[bomoko]

This POC introduces the ability to pass build variables to a deployment in a way that mirrors the GraphQL API.

It adds the ability to add command line arguments of the form --buildvar KEY1=VAL1 [--buildvar KEY2=VAL2] etc.

These will then be passed along to the graphQL deploy call as build variables.

This functionality is particularly important when deploying a new environment that requires build variables, which cannot be set since the environment doesn't yet exist.

Checklist

• Affected Issues have been mentioned in the Closing issues section
• Documentation has been written/updated.
• Changelog entry has been written

[shreddedbacon]

This is good. The split after 2 would be good to address though before merge in case someone adds a base64 encoded variable payload that contains potentially additional = signs.

[shreddedbacon]

Looks good now. SplitN 👨‍🍳