feat: Shared Mobile API Client

[PlainOlSoapBar]

Summary

Resolves #6:
Create a standardized API for writing/calling backend requests and throwing API-related errors.

New files:

• src/services/api/apiClient.ts - ApiClient implementation.
• src/services/api/apiError.ts - ApiClient's AppError definitions and class constructor.
• src/types/apiClient.types.ts - Type & interface definitions for ApiClient.
• src/docs/api-client.md - Documentation on ApiCilent's types, interfaces, and AppErrors.

Developers can create ApiClients via createApiClient() and passing in a bodyUrl, an optional tokenProvider, and timeoutMs.

Using this client, they can call the four GET/POST/PUT/DELETE requests. These requests can be aborted by timeout or manual cancellation.

Tests

Run the new tests with: npm run test -- ./test/apiClient.test.ts.
This should be result:

[Copilot]

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[PlainOlSoapBar]

I didn't even mean to add copilot as a reviewer lol, and now I can't remove it :V