oauthex: move internal/oauthex to oauthex (#559)

This PR relocates oauthex and moves DCR logic into dcr.go.

Fixes #558

oauthex/testdata/client-auth-meta.json

@@ -0,0 +1,8 @@
+{
+    "client_name": "My Test App",
+    "redirect_uris": [
+        "https://client.example.org/callback",
+        "https://client.example.org/callback2"
+    ],
+    "scope": "read write"
+}

oauthex/testdata/google-auth-meta.json

@@ -0,0 +1,57 @@
+{
+ "issuer": "https://accounts.google.com",
+ "authorization_endpoint": "https://accounts.google.com/o/oauth2/v2/auth",
+ "device_authorization_endpoint": "https://oauth2.googleapis.com/device/code",
+ "token_endpoint": "https://oauth2.googleapis.com/token",
+ "userinfo_endpoint": "https://openidconnect.googleapis.com/v1/userinfo",
+ "revocation_endpoint": "https://oauth2.googleapis.com/revoke",
+ "jwks_uri": "https://www.googleapis.com/oauth2/v3/certs",
+ "response_types_supported": [
+  "code",
+  "token",
+  "id_token",
+  "code token",
+  "code id_token",
+  "token id_token",
+  "code token id_token",
+  "none"
+ ],
+ "subject_types_supported": [
+  "public"
+ ],
+ "id_token_signing_alg_values_supported": [
+  "RS256"
+ ],
+ "scopes_supported": [
+  "openid",
+  "email",
+  "profile"
+ ],
+ "token_endpoint_auth_methods_supported": [
+  "client_secret_post",
+  "client_secret_basic"
+ ],
+ "claims_supported": [
+  "aud",
+  "email",
+  "email_verified",
+  "exp",
+  "family_name",
+  "given_name",
+  "iat",
+  "iss",
+  "name",
+  "picture",
+  "sub"
+ ],
+ "code_challenge_methods_supported": [
+  "plain",
+  "S256"
+ ],
+ "grant_types_supported": [
+  "authorization_code",
+  "refresh_token",
+  "urn:ietf:params:oauth:grant-type:device_code",
+  "urn:ietf:params:oauth:grant-type:jwt-bearer"
+ ]
+}