Previously, Fluentd users had to use the --umask command-line argument
to set the umask, which was inconvenient for service deployments and
containerized environments. This commit introduces a umask option in
the Fluentd system configuration, allowing users to define umask
directly within fluent.conf.

Additionally, this commit fixes test cases related to umask handling:
- The previous tests incorrectly asserted the expected umask values.
- Adjusted tests to ensure that new file permissions are correctly
  applied according to the configured umask.
- Added a test to verify that an invalid umask value raises the expected
  Fluent::ConfigError.

Fixes fluent#4816

Release Note:
- Added umask option to Fluentd system configuration.

Signed-off-by: kushynoda <egemen.utku3@gmail.com>