provider: convert `user_agent` argument to list of strings

[jar-b]

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Description

For parity with the standard provider and the provider_meta schema slated to be added with #474, the user_agent schema should change to a list of strings in the v2.0.0 release. This is a breaking change, and unfortunately doesn't allow for migration prior to v2.0.0 as the argument name must be kept the same. We can potentially deprecate the existing argument, but a warning diagnostic when set might be more appropriate as the argument won't be removed.

We also might want to consider tackling hashicorp/aws-sdk-go-base#1397 as part of this change, given the current provider argument is the final upstream dependency relying on the UserAgentProduct data structure.

Potential Terraform Configuration

provider "awscc" {
  user_agent = [
    "example-module/1.0",
    "BuildID/1234",
  ]
}

References

Relates #474 (comment)
Relates hashicorp/terraform-provider-aws#45464
Relates hashicorp/aws-sdk-go-base#1397