Add a benchmark to test fetching of many relationships.

[mattalbr]

This functions as a baseline, to confirm a bug that I believe I spotted which effectively serializes resolution of each relationship.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #49 will create unknown performance changes

_{Comparing mattalbr:benchmark_relationships (c1382fd) with main (9746996)}

Summary

🆕 2 new benchmarks ⁉️ 1 dropped benchmarks

:warning: _Please fix the performance issues or acknowledge them on CodSpeed._

Benchmarks breakdown

	Benchmark	main	mattalbr:benchmark_relationships	Change
🆕	test_load_many_relationships_async[postgresql-postgresql]	N/A	286 ms	N/A
🆕	test_load_many_relationships[postgresql]	N/A	199.3 ms	N/A
⁉️	test_hello_world	27.7 µs	N/A	N/A

[codecov-commenter]

Codecov Report

Merging #49 (c1382fd) into main (070ec9a) will increase coverage by 4.75%. Report is 3 commits behind head on main. The diff coverage is 98.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #49 +/- ## ========================================== + Coverage 77.37% 82.13% +4.75% ========================================== Files 10 10 Lines 725 907 +182 Branches 108 125 +17 ========================================== + Hits 561 745 +184 + Misses 135 129 -6 - Partials 29 33 +4 ```