fix: candid spec to support setting multiple custom HTTP headers on asset creation

[smallstepman]

Description

Continuation of #282. The previous PR incorrectly defined new field in candid spec, which resulted in an incorrect generation of types in e.g. TS:

export interface CreateAssetArguments {
    key: Key;
    content_type: string;
    headers: [] | [HeaderField];
    max_age: [] | [bigint];
}
export declare type HeaderField = [string, string];

This PR fixes that, helping generate correct types (again, TS as an example):

export interface CreateAssetArguments {
  'key' : Key,
  'content_type' : string,
  'headers' : [] | [Array<HeaderField>], // fixed
  'max_age' : [] | [bigint],
}
export type HeaderField = [string, string];

How Has This Been Tested?

manually inspected if generated TS types from candid spec are correct

cargo install --git https://github.com/dfinity/candid didc
didc bind src/ic-certified-assets/assets.did -t ts

Checklist:

• The title of this PR complies with Conventional Commits.
• I have edited the CHANGELOG accordingly.
• I have made corresponding changes to the documentation.

[viviveevee]

Suggested change

	- headers field in Candid spec accepts mmultiple HTTP headers
	- headers field in Candid spec accepts multiple HTTP headers