Refactor select tokens logic

[findolor]

Motivation

See issue: #1135

We needed to change how select tokens work in our GUI bindings: #1148 and #1149. With this change, we needed to make token field in Gui and Order structs optional, enabling yaml parsing without parsing tokens.

However we still need to have tokens during calldata generation so there are errors that prevent missing tokens during these operations.

Solution

Token Struct

• Added methods to add and remove token record to the yaml string

Select Tokens

• This field can now be used to directly manipulate our yaml string. It can be used to add and remove tokens
• The tokens that are listed in select tokens do not have to be in yaml from the start
• If a token is referenced in order but not present in the select token list, this throws an error

Context

• Added select tokens to the context so that we can check our tokens properly in order struct

Checks

By submitting this for review, I'm confirming I've done the following:

• made this PR as small as possible
• unit-tested any new functionality
• linked any relevant issues or PRs
• included screenshots (if this involves a front-end change)