Use CosmosSdk as the chain type if omitted, for backward compatibility

[romac]

This PR changed the ChainConfig type to an enum to allow for settings which are specific to a type of chain (eg. Penumbra, Namada, CosmosSdk), but that had the effect of making the type key in the [[chain]] config required. So when we release this as is, it will require all operators to update their config to add type = "CosmosSdk".

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To avoid breaking operators' configurations, we need to make the CosmosSdk the default value for the type setting.

Doing this requires a bit of hack, due to serde's lack of support for specifying a default variant when a tag is not provided.

See the doc comment on the ChainConfig struct for more details.

The idea is to use an untagged enum, but to ensure that each underlying variant has a type field which can only be deserialized from a single value. For this, the mono state crate provides a MustBe! macro which expands into a type that can only be deserialized from the given string, eg. MustBe!("Hello") can only be deserialized from the string Hello.

With this in place, we can make one of the variant the default one by annotating its type field with #[serde(default)].

In a nutshell:

use monostate::MustBe;
use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize, PartialEq, Eq)]
pub struct CosmosSdkConfig {
    #[serde(default)]
    r#type: MustBe!("CosmosSdk"),

    id: String,
}

#[derive(Debug, Serialize, Deserialize, PartialEq, Eq)]
pub struct NamadaConfig {
    r#type: MustBe!("Namada"),

    id: String,
}

#[derive(Debug, Serialize, Deserialize, PartialEq, Eq)]
#[serde(untagged)]
enum ChainConfig {
    CosmosSdk(CosmosSdkConfig),
    Namada(NamadaConfig),
}

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PR author checklist:

• Added changelog entry, using unclog.
• Added tests: integration (for Hermes) or unit/mock tests (for modules).
• Linked to GitHub issue.
• Updated code comments and documentation (e.g., docs/).
• Tagged one reviewer who will be the one responsible for shepherding this PR.

Reviewer checklist:

• Reviewed Files changed in the GitHub PR explorer.
• Manually tested (in case integration/unit/mock tests are absent).