roles and scripts metadata

After this ticket, document https://github.com/tarantool/doc/issues/5179

Related dev. issue(s): https://github.com/tarantool/tarantool/issues/10182
Product: Tarantool
Since: 3.4
Root document: https://www.tarantool.io/en/doc/latest/reference/configuration/configuration_reference/#app
SME: @ grafin

Details

Metadata can be specified for roles and scripts (module or file in app
config section) in lua files. To add metadata to a source file, a
comment must be added to the beginning of the file (or after a shebang).
The metadata comment must contain a YAML-formatted section begging with
'--- #tarantool.metadata.v1' line and ending with '...' line. Example:

-- This script will be loaded before the first box.cfg() call.
-- --- #tarantool.metadata.v1
-- early_load: true
-- ...
-- More comments can be added here.

Currently only 'early_load' tag is supported. Setting 'early_load' tag
makes it so that the tagged file will be loaded (required) before the
execution of box.cfg by the config module. During the preload phase
config:get() will not work, both for modules and app scripts.

Requested by @ grafin in https://github.com/tarantool/tarantool/commit/4f592623a8b82e88b701d4f289019727e330d117.

Open the configuration reference's app section linked as the root document and review the metadata details in this issue. Document the YAML metadata block syntax, the early_load tag, its preload behavior, and the config:get() limitation for modules and app scripts. Done means the app configuration documentation covers these roles and scripts metadata rules.