[WIP] Refactor `stellar-etl` to use GCS backend instead of Captive Core

[lucaszanotelli]

This PR implements the new libraries metaarchive and support/storage from stellar/go@lighthorizon to make use of GCS as a backend instead of the resource hungry Captive Core.

The following export commands were refactored:

• export_transactions
• export_effects
• export_operations
• export_assets
• export_trades
• export_ledger_entry_changes

There's still some work to do:

• Remove hardcoded bucket (will it be filled with the --testnet logic?)
• Update all tests to run against public ledgers (maybe after LedgerExporter is up and running?)

Note: the reason behind args "--gcs-bucket", "not" inside tests is to prevent cmd from deleting the file. Since I have valid credentials for the GCP project and the default bucket is a valid bucket, the cmd uploads the file to GCS and delete from my machine which ultimately causes the test to fail locally.

[jensenity]

looking forward for this PR to merge tho!

[sydneynotthecity]

Closed completed in #235