Configurable SSO directories

Allows configuring SSO directories explicitly in addition to more sane defaults (discovering them from AWS SDK credentials/config file locations)

lib/aws_sso_lib/sso.py

@@ -39,10 +39,23 @@
 
 SSO_TOKEN_DIR = os.path.expanduser(
     os.path.join("~", ".aws", "sso", "cache")
+
+AWS_CREDENTIALS_DIR = (
+    os.path.dirname(os.environ.get("AWS_SHARED_CREDENTIALS_FILE", "")) or 
+    os.path.dirname(os.environ.get("AWS_CONFIG_FILE", "")) or
+    os.path.expanduser(os.path.join("~", ".aws"))
+)
+
+SSO_TOKEN_DIR = os.environ.get("AWS_SSO_UTIL_SSO_TOKEN_DIR") or os.path.join(
+    AWS_CREDENTIALS_DIR,
+    "sso",
+    "cache",
 )
 
-CREDENTIALS_CACHE_DIR = os.path.expanduser(
-    os.path.join("~", ".aws", "cli", "cache")
+CREDENTIALS_CACHE_DIR = os.environ.get("AWS_SSO_UTIL_CREDENTIALS_CACHE_DIR") or os.path.join(
+    AWS_CREDENTIALS_DIR,
+    "cli",
+    "cache",
 )
 
 LOGGER = logging.getLogger(__name__)