Set box.cfg table value via env vars

This patch implements a way to set tables as box.cfg options value in
two different ways:

• as plain key-value table:

bash> export TT_LOG_MODULES=aaa=info,bbb=error && ./tarantool
Tarantool 2.10.0-beta1-1977-g2970bd57a
type 'help' for interactive help
tarantool> box.cfg{}
...
2023-04-11 07:22:10.951 [219020] main/103/interactive/box.load_cfg I> set \
'log_modules' configuration option to {"aaa":"info","bbb":"error"}
---
...

tarantool> box.cfg.log_modules.aaa
---
- info
...

tarantool> box.cfg.log_modules.bbb
---
- error
...

• as a table in json encoding (important: don't forget to put env var
  value into single quotes):
  NO_WRAP

bash> export TT_METRICS='{"labels":{"alias":"mystorage"},"include":"all","exclude":["vinyl"]}' && ./tarantool
Tarantool 2.10.0-beta1-1977-g2970bd57a
type 'help' for interactive help
tarantool> box.cfg{}
...
2023-04-11 07:26:03.635 [219288] main/103/interactive/box.load_cfg I> set \
'metrics' configuration option to {"exclude":["vinyl"],"include":"all",\
"labels":{"alias":"mystorage"}}
---
...

tarantool> box.cfg.metrics.include
---
- all
...

NO_WRAP
Requested by @Lord-KA in https://github.com/tarantool/tarantool/commit/cd58c321bc494d0ccac3350cf373cec6cf0f5fd9.

Start at the box.cfg configuration documentation and locate the section describing environment-variable options. Document both plain key-value tables and JSON-encoded tables, including the examples shown here; done means readers can configure and verify table-valued options through environment variables.