Add support for all streams

[theViz343]

What does this PR do, and why?

This PR adds support for unsubscribed streams and never_subscribed streams in ZT. It creates new data structures and accessor methods to access them. This PR is needed for further work on reducing stream related issues and adding support for stream (un)subscription.

Outstanding aspect(s)

External discussion & connections

• [x] Discussed in #zulip-terminal in Adding streams throws key error
• [ ] Fully fixes #
• [x] Partially fixes issue #816
• [ ] Builds upon previous unmerged work in PR #
• [ ] Is a follow-up to work in PR #
• [ ] Requires merge of PR #
• [x] Merge will enable work on #1387

How did you test this?

• [ ] Manually - Behavioral changes
• [ ] Manually - Visual changes
• [x] Adapting existing automated tests
• [x] Adding automated tests for new behavior (or missing tests)
• [ ] Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• [x] It is a minimal coherent idea
• [x] It has a commit summary following the documented style (title & body)
• [x] It has a commit summary describing the motivation and reasoning for the change
• [x] It individually passes linting and tests
• [x] It contains test additions for any new behavior
• [x] It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual changes

[Subhasish-Behera]

@theViz343 Thanks for working on this! This seems to work well and was pretty easy to understand because of the commit structure and commit messages. Apart from that, I have some minor code suggestions in the comments.

[zulipbot]

Heads up @theViz343, we just merged some commits that conflict with the changes you made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/main branch and resolve your pull request's merge conflicts accordingly.