manageBuyOffer inconsistent parameter naming

[MisterTicot]

In js-stellar-base, manageBuyOffer amount parameter has been named buyAmount.

However, in the stellar.org guide it is referenced as amount. Horizon uses amount parameter as well when returning the operation record.

Hence, I assume js-stellar-base should use amount instead of buyAmount for a matter of consistency, which is also more consistent with manageSellOffer & passiveSellOffer amount parameters.

[MisterTicot]

The sooner this one is dealt with, the better: the last thing we want is inconsistent naming to accumulate over time.

[Ryang-21]

The XDR definition names the object field this so the sdk stays consistent with it. The XDR is viewed as the source of truth and IMO it reflects more accurately what the field represents.