[Hardhat] Use `blockNumber` configuration setting to fetch `allowances` and token associations

[HIP-980](https://hips.hedera.com/hip/hip-980) introduces a `timestamp` parameter that allows to query `allowances` and token associations up to a given `timestamp`.

Whenever the user specifies a `blockNumber` in their configuration setting, we should fetch allowances and token associations up to that specified `blockNumber`. The conversion from `blockNumber` to `timestamp` was introduced in issue #77.

> [!NOTE]
> This issue is blocked until HIP-980 is implemented in the Mirror Node and deployed to `mainnet`, `testnet` and `previewnet`.