Provide a way for adding custom metadata into the ABI

[miraclx]

The ABI spec allows arbitrary metadata;

/// Other arbitrary metadata.
#[serde(default, flatten, skip_serializing_if = "HashMap::is_empty")]
pub other: HashMap<String, String>,

I was thinking cargo-near could provide a way for users to seamlessly populate this entry, something like -M, --metadata that takes an =-delimited key-value pair, and is repeatable. Optionally, this could also take a comma-separated list of values for convenience.

Something like:

$ cargo near build --metadata a=b --metadata c=d
$ cargo near build -M a=b,c=d -M e=f

I personally think this is helpful, but I want to hear what you guys think about this.

[itegulov]

I think this might be helpful, but I would rather wait for a user-driven demand to appear first to understand the use pattern. Unless you already have something specific in mind?