tt: provide jsonschemas for `tt.yaml` and `instances.yaml`

[georgiy-belyanin]

JSON Schemas allows one to define the structure of a JSON (or YAML in our case) document and statically validate its content.

Currently, there is a Tarantool JSON schema simplifying it for the user to work with Tarantool 3.x cluster configuration. E.g. it might be used within Visual Studio Code by setting up a Red-hat YAML extension.

It'd nice to have a similar schema for tt.yaml and instances.yaml for user to be able to statically validate tt specific configuration. Its configuration is less subject to change than the cluster configuration though it also means it can be simply put into the repository without the need of generating it by sources as it's done in Tarantool.