config module `has_role`, `is_router` and `is_storage` methods

Related dev. issue(s): https://github.com/tarantool/tarantool/issues/11288
Product: Tarantool
Since: 3.4
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/config/#lua-object.config
SME: @ grafin

Details

has_role(<role_name>, {instance = <instance_name}) returns true if
the instance with the name <instance_name> has role <role_name> enabled
in the current config, false if not. Second argument is optional, and
if not provided the check will be done for the instance the method is
called on.

is_router({instance = <instance_name}) returns true if the instance
with name <instance_name> is a vshard router, according to the current
config, false if not. Argument is optional, and if not provided the
check will be done for the instance the method is called on.

is_storage({instance = <instance_name}) returns true if the instance
with name <instance_name> is a vshard storage, according to the current
config, false if not. Argument is optional, and if not provided the
check will be done for the instance the method is called on.
Requested by @ grafin in https://github.com/tarantool/tarantool/commit/76c82641577fed83bada247a9a9168d5d9204e99.

Start with the linked root document at reference/reference_lua/config/#lua-object.config and add the three config methods described in this issue. Done means the documentation explains each method, its optional instance argument, and its return value for the current configuration; no test file is mentioned in the issue.