feat: add get_genesis_block rpc method

[codecov-commenter]

Codecov Report

Base: 76.81% // Head: 76.75% // Decreases project coverage by -0.05% :warning:

Coverage data is based on head (a51d50f) compared to base (0ae7014). Patch coverage: 25.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #78 +/- ## =========================================== - Coverage 76.81% 76.75% -0.06% =========================================== Files 23 23 Lines 4205 4208 +3 =========================================== Hits 3230 3230 - Misses 975 978 +3 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `76.75% <25.00%> (-0.06%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nervosnetwork#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/nervosnetwork/ckb-light-client/pull/78?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nervosnetwork) | Coverage Δ | | |---|---|---| | [src/service.rs](https://codecov.io/gh/nervosnetwork/ckb-light-client/pull/78/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nervosnetwork#diff-c3JjL3NlcnZpY2UucnM=) | `65.70% <0.00%> (-0.26%)` | :arrow_down: | | [src/storage.rs](https://codecov.io/gh/nervosnetwork/ckb-light-client/pull/78/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nervosnetwork#diff-c3JjL3N0b3JhZ2UucnM=) | `94.38% <100.00%> (ø)` | | 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=nervosnetwork). 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=nervosnetwork)

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