Add support for variable TCP-header options

## Background

As per [IETF RFC #793](https://tools.ietf.org/html/rfc793#section-3.1)'s *Options* field:

> Options may occupy space at the end of the TCP header and are a multiple of 8 bits in length. All options are included in the checksum. An option may begin on any octet boundary. There are two cases for the format of an option:
> * Case 1: A single octet of option-kind.
> * Case 2: An octet of option-kind, an octet of option-length, and the actual option-data octets.
>
> The option-length counts the two octets of option-kind and option-length as well as the option-data octets.
>
> Note that the list of options may be shorter than the data offset field might imply. The content of the header beyond the End-of-Option option must be header padding (i.e., zero).

> Currently defined options include (kind indicated in octal):

> | Kind | Length | Meaning |
> | -----|:-------:| -----:|
> | 0 | - | End of option list |
> | 1 | - | No-Operation |
> | 2 | 4 | Maximum Segment Size |

More about these definitions can be found in the spec linked.

Currently, we've left this as a [*TODO*](https://github.com/capsule-rs/capsule/blob/master/core/src/packets/tcp.rs#L179) in working with the data offset portion of the TCP header.

## Solution?

In dealing with parsing dynamic options, we've done similar work w/ [NDP options](https://github.com/capsule-rs/capsule/blob/426670213b6259ac1fd774023f7919437baa21f0/core/src/packets/icmp/v6/ndp/options/mod.rs), but we need to figure out what the best fit is here for handling these specific options from the spec.

Start in core/src/packets/tcp.rs at the TODO around the data-offset handling, then compare the dynamic parsing approach in core/src/packets/icmp/v6/ndp/options/mod.rs. Use RFC 793 to define the supported TCP option forms and padding behavior; done means variable-length options are parsed consistently with the header's data offset.

Written by the indexing model from the issue text.