feat(transport)!: replace axios with native fetch

[tharropoulos]

Change Summary

• add native fetch-based Transport class for making HTTP requests
• replace axios with fetch throughout the client library (breaking change)
• add fetchMock test utility for mocking fetch in tests
• update all test suites to work with fetch-based requests
• update examples to use fetch-based transport
• remove axios from package.json dependencies

PR Checklist

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