feat: Support for tracing ID headers

[Anush008]

Built on top of #125

const results = await withHeaders({ 'x-request-id': 'abc-123' }, () =>
    client.query(...)
);

// Nesting merges headers
withHeaders({ 'x-request-id': 'outer' }, () => {
    withHeaders({ 'x-tracing-id': 'inner' }, () => {
        // both x-request-id and x-tracing-id are sent
        client.query(...);
    });
});

[Anush008]

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