Add installment credit support (#8)

* feat: add installment credit functionality
* test: replicate installment tests from common
* fix: whitespaces
* test: add missing installment tests
* feat: update `InstallmentCreditStatusChanged` event
* feat: rename `CreditAgent_InvalidInputArrays` error
* feat: update `InstallmentCreditStatusChanged` event
* feat: rename `common` to `ordinary`
* feat: update `_sumArray` function
* fix: `initiateInstallmentCredit` function
* feat: update `AgentState` struct
* feat: update `_toUint256Array` functions
* feta: add `INSTALLMENT_COUNT_STAB` constant
* test: fix formating issues, reorder constants alphabetically
* test: improve test section names
* feat: update contract version to `v1.2.0`
* feat: update `InstallmentCreditStatusChanged` event

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Co-authored-by: Evgenii Zaitsev <evgenii@cloudwalk.io>