box: introduce space:stat()

See the doc bot request for the description.

Benchmark results:

NO_WRAP
$ taskset 0x2 compare.py benchmarks ./memtx.perftest.old ./memtx.perftest.new \
  --benchmark_min_warmup_time=10 \
  --benchmark_repetitions=30 \
  --benchmark_report_aggregates_only=true \
  --benchmark_filter=TreeReplaceRandomExistingKeys
[...]
Comparing ./memtx.perftest.old to ./memtx.perftest.new
Benchmark                                                           Time             CPU      Time Old      Time New       CPU Old       CPU New
------------------------------------------------------------------------------------------------------------------------------------------------
MemtxFixture/TreeReplaceRandomExistingKeys_mean                  +0.0097         +0.0097          1073          1084          1073          1084
MemtxFixture/TreeReplaceRandomExistingKeys_median                +0.0075         +0.0075          1062          1070          1062          1070
MemtxFixture/TreeReplaceRandomExistingKeys_stddev                -0.1207         -0.1208            56            49            56            49
MemtxFixture/TreeReplaceRandomExistingKeys_cv                    -0.1291         -0.1292             0             0             0             0
NO_WRAP

Closes #6762

@TarantoolBot document
Title: Document `space:stat()`
Product: Tarantool
Since: 3.0
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/

space_object:stat() - Get statistics on the memory usage

Returns a table with the cumulative statistics on the memory usage by
tuples in the space. The statistics is grouped by arena types: "memtx"
or "malloc".
For a detailed description of each field see `tuple_object:info()`.

Note: The statistics is collected only for memtx storage engine. For other
types of spaces, an empty table is returned.

Example:

```
tarantool> box.space.test:stat()
---
- tuple:
    memtx:
      data_size: 5100699
      header_size: 96
      field_map_size: 40
      waste_size: 143093
    malloc:
      data_size: 18850077
      header_size: 70
      field_map_size: 28
      waste_size: 0
...
```