fix: ASPA JSON deserialization failing on ASNs

[ember-ana]

As far as I could tell, there is no consistent spec for how ASPA should be represented as JSON. Here are some examples i found in the wild:

type JsonAspaRpkiClient {
	customer_asid: number,
	providers: number[],
}

type JsonAspaRoutinator {
	customer: string, // "AS64500"
	providers: string[],
}

type JsonAspaKrill {
	customer: number,
	providers: number[]
}

#132 implements serialization to rpki-client notation, as well as serialization from rpki-client notation.

Unfortunately, this means that e.g. using RTRTR with Routinator leads to errors such as Failed parsing source: invalid type: string "AS64500", expected u32 or Failed parsing source: missing field 'customer_asid'.

This PR implements support for deserializing all 3 notations, and changes serialization to krill notation, as it's the one with the most common ground between all, and therefore has the highest likeliness of a client being able to understand it.

[partim]

Thank you for the PR – being flexible here certainly makes sense. The PR looks good – apart from whatever Clippy is complaining about.

@devsnek – the PR changes the output formatting of ASPAs you defined in #132. I think the new format makes more sense (for one, underscore names in JSON feel wrong to me). I hope this is okay for you?

[devsnek]

this is fine, I was trying to copy other implementations but I guess I wasn't broad enough.

[ember-ana]

resolved clippy issues :)