Support JSON tags for nullable enum structs

[mikemherron]

Add JSON tags on to generated null enum structs. Resolves #2115.

However, while this works, it may be better implemented as a dedicated config option because:

1. The current implementation only honours the emit_json_tags option. The emit_db_tags is ignored, as it doesn't really make sense in this context since the struct does not represent a database entity. I think this is a logical descision, but it may initially be confusing to users why the null enums have one type of tag but not the other.
2. In the case of json_tags_case_style being "none", the tag name should fall back to the database column name. However, for the Valid property of the Null enum structs there is no corresponding database column, so "valid" will always be used which might be inconsistent with other fields.
3. This could also be considered a breaking change as sqlc will now add JSON tags in to structs that did not have them previously (and would have been serialised/deserialised using default behaviour defined by Go).

An alternative to this (which I am happy to implement) would be to introduce a new config option, documented as so:

emit_json_tags_on_enum_structs - If true, and emit_json_tags is also true, add JSON tags on to generated null enum structs. Defaults to false. When true, the case style of the tags will follow that defined by json_tags_case_style, unless it is none, in which case the Valid property will use the tag format valid.

Which resolves the issues I raised. It feels a bit clunky, but I think it is clearer, and keeps the default behaviour as-is while supporting niche use-cases.

Opened the PR for feedback; I will wait for maintainer feedback before starting on the additional config option.

[mikemherron]

Sorry for the tag @kyleconroy but do you have an opinion on this one? If you are in agreement with my config proposal, I have time over the weekend to implement this and update the PR. If you'd rather leave this feature out entirely, no problem, feel free to close the issue.

[kyleconroy]

@mikemherron Apologies for not getting to this one sooner.

I think your last point is correct. This could be a breaking change for users, as the JSON format of nullable enums will change. Let's go with your second plan of adding a new configuration option to control this behavior.

[mikemherron]

@mikemherron Apologies for not getting to this one sooner.

I think your last point is correct. This could be a breaking change for users, as the JSON format of nullable enums will change. Let's go with your second plan of adding a new configuration option to control this behavior.

No problem @kyleconroy, I should have time to do this over the next week.

[mikemherron]

Unfortunately, EmitJsonTagsOnNullEnumStructs is the longest config property name so causes go fmt to realign all the config structs, making the PR look a bit busier than it should in places like this.

[mikemherron]

JSON tags on null enum structs are now off by defaullt, and must be turned on with the emit_json_tags_on_enum_structs config option. PR is ready to review again.

[kyleconroy]

After thinking about it a bit more, I decided that the extra configuration option isn't worth it. If emit_json_tags is on, we'll add the tags to nullable enums. I cleaned up the code slightly as well.