Add github webhook event type

[armenzg]

This PR introduces comprehensive TypedDict definitions for GitHub webhook events in src/sentry/integrations/github/webhook_types.py.

The primary goal is to enhance type safety, improve developer experience with better autocomplete, and provide clearer documentation of expected webhook payload structures within the GitHub integration.

Key changes include:

• Defining detailed TypedDict classes for common GitHub objects (e.g., GitHubUser, GitHubRepository, GitHubPullRequest) and specific webhook event types (e.g., PushWebhookEvent, PullRequestWebhookEvent).
• Updating src/sentry/integrations/github/webhook.py to leverage these new types.
• Ensuring mypy passes by strategically using Mapping[str, Any] in public method signatures (to maintain compatibility with parent classes) and casting to specific TypedDict types internally for more precise type checking.
• The changes are backward compatible and do not alter runtime behavior, focusing solely on static type analysis benefits.

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

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[codecov]

❌ 2 Tests Failed:

Tests completed	Failed	Passed	Skipped
30203	2	30201	240

View the top 2 failed test(s) by shortest run time

tests.sentry.integrations.github.test_webhooks.InstallationEventWebhookTest::test_installation_created

Stack Traces | 0.296s run time

#x1B[1m#x1B[.../integrations/github/test_webhooks.py#x1B[0m:123: in test_installation_created
    assert integration.metadata["sender"]["id"] == 1
#x1B[1m#x1B[31mE   AssertionError: assert '1' == 1#x1B[0m

tests.sentry.integrations.github.test_installation.InstallationEndpointTest::test_installation_endpoint

Stack Traces | 0.422s run time

#x1B[1m#x1B[.../integrations/github/test_installation.py#x1B[0m:55: in test_installation_endpoint
    assert body["sender"]["id"] == 1
#x1B[1m#x1B[31mE   AssertionError: assert '1' == 1#x1B[0m

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