feat: extend setPayableAmount

[0xivanov]

Description:
Extends setPayableAmount function in ContractExecuteTransaction, by allowing the user to pass a HbarUnit as optional second parameter.

Related issue(s):

Fixes #3129

Notes for reviewer:

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[lfdt-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/Hbar.js	96.92% <100.00%> (ø)
src/contract/ContractExecuteTransaction.js	98.55% <100.00%> (+0.01%)	⬆️

... and 1 file with indirect coverage changes

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