New `session.users.<name>.idle_timeout` option

Since: Tarantool 3.8

A new declarative configuration option allows to close idle binary
sessions of a user automatically (by default, the feature is disabled).
session.users.<name>.idle_timeout is the idle timeout, in seconds, for
connections authenticated as the given user:

session:
  users:
    alice:
      idle_timeout: 60
    bob:
      idle_timeout: 300

Behavior:

• A connection authenticated as the given user is closed automatically
  if it receives no data from the client for longer than the timeout.
  The client must reconnect or re-authenticate to resume.
• A timeout of zero disables the mechanism for that user.
• Connections with in-flight requests are not closed by this mechanism.
• The option is dynamically changeable at runtime, though active
  connections are not affected.

Root document: https://www.tarantool.io/en/doc/latest/reference/configuration/
Requested by @drewdzzz in https://github.com/tarantool/tarantool/commit/f85582d29fe992b3e8d00158d1dfbc5f96661495.

Start with the root configuration document linked in the issue and locate the section describing per-user session options. Document session.users..idle_timeout with the example and listed runtime behavior, including the default-disabled zero value, idle connection closure, in-flight request exception, and dynamic configuration limitation.