feat: add one-shot cost-report export command

[joshrotenberg]

Summary

Adds a new export subcommand that combines generate + wait + download into a single operation for simpler cost report retrieval.

Usage

# Export monthly costs to file
redisctl cloud cost-report export \
  --start-date 2025-01-01 \
  --end-date 2025-01-31 \
  --file january-costs.csv

# Export as JSON
redisctl cloud cost-report export \
  --start-date 2025-01-01 \
  --end-date 2025-01-31 \
  --format json \
  --file january-costs.json

# With filters
redisctl cloud cost-report export \
  --start-date 2025-01-01 \
  --end-date 2025-01-31 \
  --tag team:platform \
  --subscription-type pro \
  --file team-costs.csv

Features

• All filter options from generate (subscription, database, region, tags, subscription-type)
• Configurable timeout (default 300s)
• Progress indicator during generation and download
• Supports both CSV and JSON output formats
• Outputs to file or stdout

Changes

• crates/redisctl/src/cli/cloud.rs: Added Export variant to CloudCostReportCommands
• crates/redisctl/src/commands/cloud/cost_report.rs: Implemented export_cost_report function
• Updated docs to recommend export as the primary workflow

Testing

Tested end-to-end:

$ ./target/debug/redisctl cloud cost-report export --start-date 2025-01-20 --end-date 2025-01-23 --format json --file test.json
Cost report exported to 'test.json' (167893 bytes)

Closes #592