Support column comments

[BenoitRanque]

This PR adds support for column comments

This required a change in configuration files, because we used to store column defintions as a Map<Name, DataType>

We add a new enum so we can store both data type and optional comment. This a not a breaking change because we still support parsing the older, shorthand format.

When introspecting, we still generate the shorthand format if the comment is empty.

Additionally, we derive deserialize and serialize for ClickHouseDataType, so we can include it directly in configuration file types.

However this introduces a problem: when unable to parse a datatype, the serde json error is very unhelpful.

#[serde(untagged)]
pub enum ColumnDefinition {
    ShortHand(ClickhouseDataType),
    LongForm {
        data_type: ClickhouseDataType,
        comment: Option<String>,
    },
}

The issue is that if serde is unable to parse the ClickHouseDataType, it reports it was unable to match any of the untagged variants. So the error is very unhelpful to users.

The solution is the MaybeClickHouseDataType enum:

#[serde(untagged)]
pub enum MaybeClickhouseDataType {
    Valid(ClickHouseDataType),
    Invalid(String),
}

The Invalid variant is the result of being unable to parse the datatype. We can then handle this gracefully and tell the user exactly what the problem is, and where.

[danieljharvey]

🎸