Add Stellar support

[jzethar]

This PR contains modules for Stellar blockchain: main module and operations module

[Har01d]

There are some issues:

• We don't seem to be taking source address into account. For example, for transaction 166798028ca757a82f6ae3e9b83eb16309a961a72fe682db7643cbe68c8f39f7 nothing is being returned for GA4SDHEDDRLOLR73QKBIWRP3SO4C7XZD6QIYQE67OYG7ALFXPEM5RRQA (note that the fee payer and the source are two different addresses for this transaction). So we need to make two extra events for the source address and a special address (the-void or operations?)
• Fees are going into the-void, but there are no reward transactions. We should either add them, or use the validator address instead of the-void (like we do in EVM modules)
• The two modules should be merged (?)

[Har01d]

Improvements are required.

[Har01d]

xlm shouldn't be hard-corded in top-level modules.

[Har01d]

Missing extra_data_model and extra_data_details.

[Har01d]

These two modules should be merged into a single one as the main module doesn't process anything besides fees.

[Har01d]

Closing in favour of #99.