Add new lookup methods for Services and Characteristics

[bdraco]

• Services.iid_or_none - fetch by service by iid or return None
• Accessories.aid_or_none - fetch accessory by aid or return None

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (c244405) 67.99% compared to head (c333f9a) 74.46%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #337 +/- ## ========================================== + Coverage 67.99% 74.46% +6.46% ========================================== Files 71 99 +28 Lines 6781 9238 +2457 ========================================== + Hits 4611 6879 +2268 - Misses 2170 2359 +189 ``` | [Files](https://app.codecov.io/gh/Jc2k/aiohomekit/pull/337?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [aiohomekit/model/\_\_init\_\_.py](https://app.codecov.io/gh/Jc2k/aiohomekit/pull/337?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-YWlvaG9tZWtpdC9tb2RlbC9fX2luaXRfXy5weQ==) | `95.73% <100.00%> (+0.30%)` | :arrow_up: | | [aiohomekit/model/characteristics/const.py](https://app.codecov.io/gh/Jc2k/aiohomekit/pull/337?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-YWlvaG9tZWtpdC9tb2RlbC9jaGFyYWN0ZXJpc3RpY3MvY29uc3QucHk=) | `100.00% <ø> (ø)` | | | [aiohomekit/model/services/service.py](https://app.codecov.io/gh/Jc2k/aiohomekit/pull/337?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-YWlvaG9tZWtpdC9tb2RlbC9zZXJ2aWNlcy9zZXJ2aWNlLnB5) | `98.80% <100.00%> (+1.05%)` | :arrow_up: | | [tests/test\_model.py](https://app.codecov.io/gh/Jc2k/aiohomekit/pull/337?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-dGVzdHMvdGVzdF9tb2RlbC5weQ==) | `100.00% <100.00%> (ø)` | | ... and [35 files with indirect coverage changes](https://app.codecov.io/gh/Jc2k/aiohomekit/pull/337/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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