feat: support custom environment prefixes (#999)

[ianatkinson-ee]

Added support for custom environment prefixes in the SDK (#999)

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[ianatkinson-ee]

@joeldodge79 the code changes here are basically just the comment here: https://github.com/looker-open-source/sdk-codegen/issues/999#issuecomment-1047886346

[joeldodge79]

well hrm, cattrs unittest are failing, seems unrelated. i'll have to dig into that for a minute