feat: add `get_open_api_specification` method to `BuildClient` #111

Job	Run time
Async dostrings check / Async docstrings check	31s
Type check / Type check (3.9)	23s
Lint check / Lint check (3.9)	20s
Docs check / Docs check	1m 35s
Type check / Type check (3.10)	30s
Lint check / Lint check (3.10)	23s
Type check / Type check (3.11)	39s
Lint check / Lint check (3.11)	19s
Unit tests / Unit tests (ubuntu-latest, 3.9)	24s
Lint check / Lint check (3.12)	20s
Lint check / Lint check (3.13)	26s
Unit tests / Unit tests (ubuntu-latest, 3.10)	29s
Type check / Type check (3.12)	24s
Unit tests / Unit tests (ubuntu-latest, 3.11)	21s
Type check / Type check (3.13)	31s
Unit tests / Unit tests (ubuntu-latest, 3.12)	24s
Unit tests / Unit tests (ubuntu-latest, 3.13)	35s
Unit tests / Unit tests (windows-latest, 3.9)	1m 13s
Unit tests / Unit tests (windows-latest, 3.10)	59s
Unit tests / Unit tests (windows-latest, 3.11)	1m 18s
Unit tests / Unit tests (windows-latest, 3.12)	2m 13s
Unit tests / Unit tests (windows-latest, 3.13)	2m 50s
Integration tests / Integration tests (3.9)	25s
Integration tests / Approve integration tests for PRs from forks	0s
Integration tests / Integration tests (3.13)	29s
Integration tests / Integration tests for PRs from forks	0s
	18m 1s

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