api: Add ZulipStream.topicsPolicy

[chungwwei]

Ingest String topics_policy field. Will be used to hide the topic input in the general-chat-only channel.

[chrisbobbe]

Thanks! Comments below.

Also, some commit-message nits:

api: Add ZulipStream.TopicsPolicy

Ingest String `topics_policy` field. The field can take be one of 4
possible values, inherit, allow_empty_topic, disable_empty_topic,
and empty_topic_only. Enum TopicsPolicy is created for these four
values.

Fixes: https://github.com/zulip/zulip-flutter/issues/1604

• ZulipStream.TopicsPolicy doesn't refer to anything that exists; instead, say ZulipStream.topicsPolicy (referring to the field) or TopicsPolicy (referring to the enum).
• The paragraph ("Ingest String…") just repeats what's already clear and expected in the diff; let's remove it, so there's less we need to read through.
• This work is a prerequisite for #1604 (making the data available to the app), but I'd like to keep #1604 open for the work of actually using the data in the compose-box UI code. So let's remove the "Fixes" line.

[chrisbobbe]

I would order this between messageRetentionDays and channelPostPolicy, here and everywhere else in this commit.

[chrisbobbe]

Also, this field is new in Zulip Server 11, per the API doc. So if we make it required with no default value, we'll throw an error when getting this data from a server older than 11.

On those older servers, do channels always behave as though they have TopicsPolicy.inherit? If so, let's use that as the default value:

Suggested change

	TopicsPolicy topicsPolicy;
	@JsonKey(defaultValue: TopicsPolicy.inherit) // TODO(server-11) remove default value
	TopicsPolicy topicsPolicy;

[chrisbobbe]

I see you've pushed changes; please comment here when this is ready for another review.

[chungwwei]

Thanks for reviewing before! It's ready for another round.

[chrisbobbe]

Thanks! Just one nit below.

[chrisbobbe]

I've also removed the "Fixes:" line from the PR description, for the same reason I mentioned above in #1956 (review).

[chrisbobbe]

I see you've pushed some updates; please comment here when this is ready for another review.

[chungwwei]

ahh I forgot to comment. Let me rebase on top of the latest since it's been awhile.

I don't think I have any more changes to add to this PR. Set up a different PR for the ui changes that requires this PR.

[chungwwei]

@chrisbobbe CI is green. Thanks for the reminder! It's ready for review..

[chrisbobbe]

Thanks for the revision! Comments below.

[chrisbobbe]

nit: too-long line

Suggested change

	static final _byApiValue = _$RealmTopicsPolicyEnumMap.map((key, value) => MapEntry(value, key));
	static final _byApiValue = _$RealmTopicsPolicyEnumMap
	.map((key, value) => MapEntry(value, key));

[chungwwei]

it's unused. I removed them

[chrisbobbe]

Instead of handling unknown values here, let's make an explicit unknown enum value, and ask the inner parts of the app code to handle that.

[chrisbobbe]

(Similarly here, let's use an explicit unknown enum value, for inward app code to handle.)

[chungwwei]

I don't think server sends random value and it's not null?
because @JsonKey(defaultValue: TopicsPolicy.inherit) is added #1956 (comment)

I believe this was the needed change.

static RealmTopicsPolicy fromApiValue(String apiValue) => _byApiValue[apiValue]!;

[chungwwei]

@chrisbobbe Thanks for reviewing! It's ready for another round.