AWS supports the creation and use of credentials that are only valid for a
fixed period of time. These credentials comprise three parts: the usual access
key and secret key, together with a session token. This commit adds support for
these three-part credentials to the EC2 discovery plugin and the S3 repository
plugin.
Note that session tokens are only valid for a limited period of time and yet
there is no mechanism for refreshing or rotating them when they expire without
restarting Elasticsearch. Nonetheless, this feature is already useful for
nodes that need only run for a few days, such as for training, testing or
evaluation. #29135 tracks the work towards allowing these credentials to be
refreshed at runtime.
Resolves#16428
4108722052