[FEATURE] Add support for extra user agent through environment variable

[osoucy]

Problem Statement
I want to append user agent information when deploying resources through Terraform (and Pulumi) Databricks provider without having to re-implement the provider. This would be required to support a partner integration and properly track relevant metrics.

Proposed Solution
Given that the terraform databricks provider uses the databricks-sdk-go user agent to define the user agent value sent to the Databricks REST API calls, adding support for an environment variable would eliminate the need for re-implementation anytime a new UserAgent value is required.

A similar implementation has already been done in the python sdk here by @nfx and @kartikgupta-db. Adding the same logic to the go sdk would resolve the stated problem.

Another, less generic, option would be to add okube-laktory to the list of official CI/CD providers defined here

Additional Context
Most terraform providers support an environment variable called TF_APPEND_USER_AGENT as described here. Unfortunately, the Terraform Databricks Provider has its own custom implementation of the user agent which prevent from using this environment variable. If I wanted to add okube-laktory to the user agent, I would have to implement custom Terraform and Pulumi Databricks providers (I need to support both). Obviously, this is quite effort-intensive and cumbersome just for the sake of adding a property to the user agent.

Another option would be to force all calls to go through a proxy which updates the headers of the API calls, but that would be a less than stellar user experience and requires to support a live service.

[nfx]

Listing it as CI provider is a better option

[osoucy]

Actually, thinking about it, Laktory (the framework I would like to have the UserAgent updated for), is more an Infrastructure as Code tool like Pulumi and Terraform than a CI/CD tool. It could possibly be used with github actions, azure-devops or others.

Given that additional context, do you still feel listing it as CI provider is the best option?