feat: add import wallet

[wenchaosong]

Description

add a method import wallet which only need privateKey and networkInfo

Checklist

• [ ] Targeted PR against correct branch.
• [ ] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• [ ] Wrote unit tests.
• [ ] Run make format.
• [ ] Run make lint.
• [ ] Updated the documentation.
• [ ] Added an entry to the CHANGELOG.md file.
• [ ] Re-reviewed Files changed in the Github PR explorer and added meaningful comments.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.31 :tada:

Comparison is base (4c4dde7) 76.39% compared to head (18151a5) 76.70%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #176 +/- ## ========================================== + Coverage 76.39% 76.70% +0.31% ========================================== Files 26 26 Lines 665 674 +9 ========================================== + Hits 508 517 +9 Misses 157 157 ``` | [Impacted Files](https://codecov.io/gh/alan-sdk/alan.dart/pull/176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alan-sdk) | Coverage Δ | | |---|---|---| | [lib/wallet/wallet.dart](https://codecov.io/gh/alan-sdk/alan.dart/pull/176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alan-sdk#diff-bGliL3dhbGxldC93YWxsZXQuZGFydA==) | `87.80% <100.00%> (+1.50%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alan-sdk). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=alan-sdk)

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[wenchaosong]

yeah, this is a useful rep, I like it