Add v3.0.0 Breaking Changes To Provider Schema

[jcudit]

Ahead of our next major release, this PR modifies the provider schema in the following ways:

• token becomes optional, with its absence signalling anonymous mode
• organization is no longer deprecated and instead enables the management of organization-only resources
• individual and anonymous removed
• owner inferred from organization when present

This also refactors the provider configuration testing into a new provider_utils file alongside the original test file. The aim was to make this suite of tests more readable.

Finally, a trivial github_organization data source was added to aid in testing.

/cc https:/terraform-providers/terraform-provider-github/issues/502#issuecomment-652399602

[jcudit]

--- PASS: TestAccConfigMeta (9.11s)
    --- PASS: TestAccConfigMeta/returns_an_anonymous_client_for_the_v3_REST_API (7.30s)
    --- SKIP: TestAccConfigMeta/returns_an_anonymous_client_for_the_v4_GraphQL_API (0.00s)
    --- PASS: TestAccConfigMeta/returns_a_v3_REST_API_client_to_manage_individual_resources (0.25s)
    --- PASS: TestAccConfigMeta/returns_a_v4_GraphQL_API_client_to_manage_individual_resources (0.20s)
    --- PASS: TestAccConfigMeta/returns_a_v3_REST_API_client_to_manage_organization_resources (0.23s)
    --- PASS: TestAccConfigMeta/returns_a_v4_GraphQL_API_client_to_manage_organization_resources (0.19s)
    --- PASS: TestAccConfigMeta/manages_individual_resources_for_a_GHES_deployment (0.94s)
PASS
ok  	github.com/terraform-providers/terraform-provider-github/github	9.789s

?   	github.com/terraform-providers/terraform-provider-github	[no test files]
=== RUN   TestAccProviderConfigure
=== RUN   TestAccProviderConfigure/can_be_configured_to_run_insecurely
=== RUN   TestAccProviderConfigure/can_be_configured_to_run_anonymously
=== RUN   TestAccProviderConfigure/can_be_configured_with_an_individual_account
=== RUN   TestAccProviderConfigure/can_be_configured_with_an_organization_account
=== RUN   TestAccProviderConfigure/can_be_configured_with_a_GHES_deployment
--- PASS: TestAccProviderConfigure (21.43s)
    --- PASS: TestAccProviderConfigure/can_be_configured_to_run_insecurely (0.17s)
    --- PASS: TestAccProviderConfigure/can_be_configured_to_run_anonymously (2.67s)
    --- PASS: TestAccProviderConfigure/can_be_configured_with_an_individual_account (5.62s)
    --- PASS: TestAccProviderConfigure/can_be_configured_with_an_organization_account (10.23s)
    --- PASS: TestAccProviderConfigure/can_be_configured_with_a_GHES_deployment (2.74s)
PASS
ok  	github.com/terraform-providers/terraform-provider-github/github	21.758s

Added more testing around the GHES use case. This is functioning well locally and will push to merge this week. Follow up PRs are expected as iteration continues towards the new acceptance testing workflow.

[jcudit]

https:/terraform-providers/terraform-provider-github/pull/544 addresses the concerns raised above.

I will merge this to accommodate progress on refreshing most tests and adding new testing workflows.

[colltoaction]

Is it possible that there is an undocumented breaking change here? When upgrading from 2.9 to 3.1 I had to change from

provider "github" {
  base_url = "https://my-ghe-host/api/v3"
  organization = "my-org"
}

to

provider "github" {
  base_url = "https://my-ghe-host"
  organization = "my-org"
}