Implement GET_SENDER protocol feature

[taokayan]

Change Description

Resolves #7028.

This PR adds support for the GET_SENDER protocol feature which allows contracts to determine which account is the sender of an inline action.

This PR introduces a new unit test protocol_feature_tests/get_sender_test to test the behavior of this new protocol feature. The get_sender_test test contract was added to meet the needs of the new unit test.

Consensus Changes

• Consensus Changes

Makes the GET_SENDER protocol feature available. See above description and #7028 for details.

API Changes

• API Changes

Documentation Additions

• Documentation Additions

[tbfleming]

After this change, there will be 2 event systems:

• require_recipient
• inline action, with receiver verifying the sender using get_sender

This brings up a question: how should they interact?

• Imagine an inline action with no args that's a simple ack.
• Intended behavior: if contract A sends an ack to contract B, contract B can verify authenticity by only looking at get_sender
• Setup:
  • A sends an ack to contract C
  • C uses require_recipient to send it to B
  • B uses get_sender
• Options:
  • get_sender returns A: B incorrectly assumes the ack was for it
  • get_sender returns 0: B doesn't fall for the attack
  • get_sender returns C: B doesn't fall for the attack

[arhag]

• Setup:

  • A sends an ack to contract C
  • C uses require_recipient to send it to B
  • B uses get_sender

• Options:

  • get_sender returns A: B incorrectly assumes the ack was for it
  • get_sender returns 0: B doesn't fall for the attack
  • get_sender returns C: B doesn't fall for the attack

This PR goes with the third option where it would return C in the situation described above.