fiber: add fiber_set_ctx & fiber_get_ctx functions

Before this change there was no way to create a fiber that accepts
parameters without yielding from the current fiber using the c api. You
could pass the function arguments when calling fiber_start, but that
forces you to yield, which is not acceptable in some scenarios (e.g.
within a transaction).

This commit introduces 2 new functions to the api: fiber_set_ctx for
setting an pointer to a context of the given fiber and fiber_get_ctx for
accessing that context.

Closes https://github.com/tarantool/tarantool/issues/7669

@TarantoolBot document
Title: fiber: add fiber_set_ctx & fiber_get_ctx functions

Add 2 api functions: `fiber_set_ctx` & `fiber_get_ctx` which can be used
for passing data to a fiber. Previously this could be done via the
`fiber_start` function, except that this would force the current fiber
to yield, which is not acceptable in some scenarios (e.g. during a
transaction). Now you can create a fiber with `fiber_new`, set it's
contents with `fiber_set_ctx`, make it ready for execution with
`fiber_wakeup` and keep executing the current fiber.