Send API key via x-api-key header instead of ?apiKey= query string

[protobi-pieter]

Migrate all authenticated requests in the R client to pass the API key in the x-api-key request header rather than appending ?apiKey= to the URL.

• R/protobi.R: all HTTP calls (get_data, put_data, run_process, execute, get_formats, get_titles, properties, table details, poll task) now send x- - protobi_get_url() / protobi_put_url() / protobi_read_csv_gzip() gained an optional apikey arg (backward-compatible).
• Example scripts updated; hardcoded keys replaced with Sys.getenv("PROTOBI_TEST_API_KEY").
• Compatibility: no public signature changes are breaking (added args are optional).