Dynamic network configuration: Provide Horizon URL and Network Passphrase

[miguelnietoa]

Context

The current approach of setting the network via application environments in the Stellar SDK needs to be revised due to the inability to support functionalities similar to Stellar Laboratory where switching between different networks is required.

Current approach is:

config :stellar_sdk, network: :test

Objective

Remove the network configuration from the envs application, and rather provide the Horizon URL and Network Passphrase.

Checklist

Horizon

• Create a Stellar.Horizon.Server module.
• Receive a Stellar.Horizon.Server struct in every module inside the Horizon boundary.

TxBuild

• Receive the network_passphase in the TxBuild struct.
• Update the Stellar.Network module to expose the passphases of each network.

[miguelnietoa]

These changes have been implemented and are available in v0.21.0.