refactor: get tokens request

[rialco]

The get tokens endpoint accepts an array of strings as the parameter for chains. It is documented here.

The issue with the current type is that Ajv JsonSchemaType will only accept numbers in the schema, stating this error:

Property 'type' is missing in type '{ oneOf: { type: string; $ref: string; }[]; }' but required in type 
'{ type: "number" | "integer"; }'.

Even with our current schema that contains chain keys and chain ids it will fail showing this error when passing a chain key:

{
    "message": "querystring/chains/0 must be integer: /chains/0: must be integer"
}

[mathiasmoeller]

Looks good!