Fix fee calculation of NativeScript transaction

[siegfried]

No description provided.

[SebastienGllmt]

nit: rename the comment in count_needed_vkeys from "minting keys" -> "native script keys"

nit: you can rename get_mint_scripts to get_native_scripts. It's fine to make breaking changes

[siegfried]

Updated. Also, how about adding a set_vkey_number() to TransactionBuilder? For 1/10 multi-sig transaction, the fee is calculated with 10 keys whereas 1 key is enough. Users could use this method to set the count explicitly when it is possible to apply a minimum fee.

[SebastienGllmt]

@siegfried yeah I think that's fine to add

we should probably document this stuff in the doc folder. The state of the documentation for this repo is terrible though so I don't blame you if you don't want to try and fix it in this PR

[siegfried]

I can do that when I have time, maybe in another PR if you want to merge this.

[SebastienGllmt]

Merging for now. Other tasks can be done as follow-up