Improved to add option for Session Token Period Second

[ggtakec]

Relevant Issue (if applicable)

n/a

Details

This library does not support session token generation(login processing) for STS/SSO, etc., so normally it is necessary to write the session token in Credentials(.aws/<file> or environment variables).
Therefore, to update the Session Token, you will need to use an external program to update the Credential.

However, it is not possible to specify an expiration date for Session Token, so the expiration date is always determined to be in the future.
Therefore, there is a problem that s3fs continues to use an expired Session Token.
(Even if s3fs determines that the Session Token has expired, there is no process to update it, so you need to notice this phenomenon and update it using another program.)

Added an option to specify the validity period(in seconds) of the Session Token in this PR.(This probably doesn't help much.)
Please see the revised README.md for details.