Environment Variables Suck

[thirtytwobits]

Many many reasons why but here are two things that need to be fixed:

1. Always recheck and rebuild the database when environment variables change. For example, I just wasted 15 minutes trying to figure out why pycyphal was trying to connect to UDP only to discover that my previous value UAVCAN__UDP__IFACE was somehow part of the db. Once I deleted this and restarted my new environment variables were found.
2. Always accept "CYPHAL_" where "UAVCAN_" is currently required. It's really hard to remember which of the environment variables are UAVCAN and which are CYPHAL. Either should be accepted but CYPHAL should be documented.

[pavel-kirienko]

1. We can make it so that environment variables override all entries in the database during initialization, like the following but executed for all records in the db during construction:

pycyphal/pycyphal/application/register/_registry.py

Lines 256 to 261 in 01b9a9b

	env_val = self.environment_variables.get(get_environment_variable_name(key))
	if env_val is not None:
	_logger.debug("%r: Update from env: %r <- %r", self, key, env_val)
	reg = self[key]
	reg.assign_environment_variable(env_val)
	self[key] = reg

2. This is outside of the scope of PyCyphal because the names come from https://github.com/OpenCyphal/public_regulated_data_types/blob/935973babe11755d8070e67452b3508b4b6833e2/uavcan/register/384.Access.1.0.dsdl#L80