config: TLS options support -- follow up

I accidentally discarded some changes made by @ImeevMA in PR #9385. The
previous version of the patchset goes to the main branch as result. I'm
very sorry.

This commit returns the code of the last version of the patchset.

The changes are described in [1]. Quoted here:

> 1. Reduce code duplication in the validation logic. It also
>    eliminates remaining (dead) code that interprets user@ and
>    user:pass@ syntax. And give better error messages in come cases
>    (comma separates URIs for example).
> 2. Forbid {} as an URI to simplify logic in instance_uri.
> 3. Refresh the comments.

[1]: https://github.com/tarantool/tarantool/pull/9385#discussion_r1430935585

Part of #8862

NO_DOC=fixup
NO_CHANGELOG=see NO_DOC