Create balance observation API endpoint

[POST] /api/balances/{address}/observation

Should remember the wallet address to observe the wallet balance and return it in the [GET] /api/balances , if the balance is non zero.

Errors:
■ 409 Conflict - specified address is already observed.

Python implementation

```py

@api.route('/balances//observation', methods=['POST'])
def add_observation(address):
"""
Add the specified address to observation list
"""

if isValidAddress(address):
result = add_address_observation(address)
else:
result = dict(error='Invalid address', status=422)

# 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 ""

```