crud: support schema

[DifferentialOrange]

Support crud.schema request [1] and response parsing.

1. https://github.com/tarantool/crud/pull/380

What has been done? Why? What problem is being solved?

I didn't forget about (remove if it is not applicable):

• [x] Tests (see documentation for a testing package)
• [x] Changelog (see documentation for changelog format)
• [x] Documentation (see documentation for documentation style guide)

[DifferentialOrange]

Will fail until crud 1.4.0 with https://github.com/tarantool/crud/pull/380 is released