Implement AssumeRole API for Minio users (#7267)

For actual API reference read here

https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRole.html

Documentation is added and updated as well at docs/sts/assume-role.md

Fixes #6381

cmd/auth-handler_test.go

@@ -381,7 +381,8 @@ func TestIsReqAuthenticated(t *testing.T) {
 	ctx := context.Background()
 	// Validates all testcases.
 	for i, testCase := range testCases {
-		if s3Error := isReqAuthenticated(ctx, testCase.req, globalServerConfig.GetRegion()); s3Error != testCase.s3Error {
+		s3Error := isReqAuthenticated(ctx, testCase.req, globalServerConfig.GetRegion(), serviceS3)
+		if s3Error != testCase.s3Error {
 			if _, err := ioutil.ReadAll(testCase.req.Body); toAPIErrorCode(ctx, err) != testCase.s3Error {
 				t.Fatalf("Test %d: Unexpected S3 error: want %d - got %d (got after reading request %s)", i, testCase.s3Error, s3Error, toAPIError(ctx, err).Code)
 			}