Refactor: split iota-transaction-builder into multiple files

[Thoralf-M]

Description

Split lib.rs in iota-transaction-builder into multiple files

Motivation

Better overview if it's not just one large file and logically separated

Requirements

1. Move request_add_stake(), request_withdraw_stake(), request_add_timelocked_stake(), request_withdraw_timelocked_stake() in a stake.rs file
2. Move select_gas(), get_object_arg(), input_refs(), resolve_and_checks_json_args(), get_object_ref(), get_object_ref_and_type() in a utils.rs file
3. Move publish_tx_kind(), publish(), upgrade_tx_kind(), upgrade() in a package.rs file

Open questions (optional)

Anyone has better suggestions for the file names?

Are you planning to do it yourself in a pull request?

No.

[sarahagib]

Hi, my name is Sarah and I am an engineering student looking for a good first issue. If no one is already working on this issue, I would like to be assigned.

[thibault-martinez]

Hi, my name is Sarah and I am an engineering student looking for a good first issue. If no one is already working on this issue, I would like to be assigned.

Hi Sarah, it's yours! Thanks :)