feat: remove credential validation on provider startup

Previously we validated that the user specified at least one kind
of Equinix credential during provider startup.  This behavior is
unusual among Terraform providers and its utility is debatable,
because it does not and cannot protect against these situations:

- Invalid credentials were provided
- Credentials were omitted for a service that is used (e.g., a Metal
  auth token was specified, but a client ID and secret were needed
  for the resources in the user's configuration)

In addition to the above shortcomings, validating credentials in
the Config type that is shared between SDKv2 and framework providers
forces unnecessary changes to the SDKv2 provider if a new service is
onboarded to the framework provider.

This removes the credential validation from the config object so
that any and all validation of the provider configuration is specified
in the provider schema.

internal/config/config.go

@@ -41,24 +41,9 @@ type ProviderMeta struct {
 }
 
 const (
-	consumerToken         = "aZ9GmqHTPtxevvFq9SK3Pi2yr9YCbRzduCSXF2SNem5sjB91mDq7Th3ZwTtRqMWZ"
-	metalBasePath         = "/metal/v1/"
-	uaEnvVar              = "TF_APPEND_USER_AGENT"
-	emptyCredentialsError = `the provider needs to be configured with the proper credentials before it
-can be used.
-
-One of pair "client_id" - "client_secret" or "token" must be set in the provider
-configuration to interact with Equinix Fabric and Network Edge services, and
-"auth_token" to interact with Equinix Metal. These can also be configured using
-environment variables.
-
-Please note that while the authentication arguments are individually optional to allow
-interaction with the different services independently, trying to provision the resources
-of a service without the required credentials will return an API error referring to
-'Invalid authentication token' or 'error when acquiring token'.
-
-More information on the provider configuration can be found here:
-https://registry.terraform.io/providers/equinix/equinix/latest/docs`
+	consumerToken = "aZ9GmqHTPtxevvFq9SK3Pi2yr9YCbRzduCSXF2SNem5sjB91mDq7Th3ZwTtRqMWZ"
+	metalBasePath = "/metal/v1/"
+	uaEnvVar      = "TF_APPEND_USER_AGENT"
 )
 
 var (
@@ -98,10 +83,6 @@ func (c *Config) Load(ctx context.Context) error {
 		return fmt.Errorf("'baseURL' cannot be empty")
 	}
 
-	if c.Token == "" && (c.ClientID == "" || c.ClientSecret == "") && c.AuthToken == "" {
-		return fmt.Errorf(emptyCredentialsError)
-	}
-
 	var authClient *http.Client
 	if c.Token != "" {
 		tokenSource := xoauth2.StaticTokenSource(&xoauth2.Token{AccessToken: c.Token})