Ability to send_periodic() a group of messages with different arbitration ids and a variable rate period

## Problem Description:
### 1. Transmit a group of messages with a Group Period and intra-Message Period
When transmitting a group of messages with `send_periodic()`, the only option is to transmit each message at a uniform `period` seconds. There are scenarios where the larger group of messages needs to have a period of `period` seconds but between each message within the group a non-uniform period of `period_intra` seconds is desired. For example, if I have a group of messages, `msg_group = [msg1, msg2]`, that needs to have a group `period` of 500 ms, however the period between `msg1` and `msg2` must be less than 100 ms then this is not possible with the current methods. I'm suggesting adding an optional `period_intra` attribute.

### 2. Transmit a group of messages with different arbitration ids
Unable to explicitly group messages with different arbitration ids to the same task. In some scenarios, it is desired to explicitly guarantee order of transmission on the bus and a defined period between messages within a group. The current implementation seems to be aimed at supporting mux'd messages only.

## Proposed Solution:
### 1. Transmit a group of messages with a Group Period and intra-Message Period
Create a child class to provide the methods to run variable rate messaging for a group of messages.
* Modify `send_periodic` method within `can/bus.py` to include an optional argument of `period_intra`
* Implement a `VariableRateCyclicTaskABC` class within `can/broadcastmanager.py` to be inherited by `ThreadBasedCyclicSendTask`
* Implement a method `_check_and_apply_period_intra()` within `VariableRateCycleTaskABC` to set variable rate attributes
* Update `ThreadBasedCyclicSendTask._run()` with variable rate attributes

### 2. Transmit a group of messages with different `arbitration_ids`
Modify `CyclicSendTaskABC` and `ModifiableCyclicTaskABC` to support a group of messages with different arbitration ids.
* Change `CyclicSendTaskABC.arbitration_id` to be a list of `arbitration_ids`
* Add `msg_index` and `msgs_len` as attributes of `CyclicSendTaskABC`
* Modify `CyclicSendTaskABC._check_and_convert_messages` and `ModifiableCyclicTaskABC._check_modified_messages` to check for specific `arbitration_id` of the modified message depending on the `msg_index` rather than just checking `message[0].arbitration_id`

## Additional context
This issue and proposed solution came from implementing updates to meet J1939-76 transmission. The ability to pass the SHM and SDM as a group into a single task is important because it explicitly links the two messages and guarantees the SHM will transmit prior to the SDM. In addition, these proposed changes allow for `period` to be set as the SDG period and the `period_intra` set as the SRVT period allowing for compliance with J1939-76 timing.