[api] Start observation from address

[POST] /api/transactions/history/from/{address}/observation

Should start observation of the transactions that transfer fund from the address . Should affect result of the `[GET] /api/transactions/history/from/{address}` .

Errors:

* `409 Conflict` : transactions from the address are already observed.

Python implementation

```py

@api.route('/transactions/history/from//observation', methods=['POST'])
def add_history_from_address(address):
"""
Starts observation of the transactions that transfer fund from the address
"""

result = add_transaction_observation_from_address(address)

# if successfully stored in observation list, return a plain 200
if "error" in result:
return make_response(jsonify(build_error(result["error"])), result["status"])
else:
return ""

```