Add missing function to read/write a `Symbol` using `LHDataStore`

[fhagemann]

readdata and writedata both were able to save Symbols using LegendHDF5IO. The new implementation with LHDataStore did not support this yet.

With this PR, reading and writing Symbols will also be supported.

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.41% :warning:

Comparison is base (3176874) 48.31% compared to head (cf9b6e5) 47.91%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #32 +/- ## ========================================== - Coverage 48.31% 47.91% -0.41% ========================================== Files 6 6 Lines 594 599 +5 ========================================== Hits 287 287 - Misses 307 312 +5 ``` | [Files Changed](https://app.codecov.io/gh/legend-exp/LegendHDF5IO.jl/pull/32?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=legend-exp) | Coverage Δ | | |---|---|---| | [src/types.jl](https://app.codecov.io/gh/legend-exp/LegendHDF5IO.jl/pull/32?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=legend-exp#diff-c3JjL3R5cGVzLmps) | `87.76% <0.00%> (-2.40%)` | :arrow_down: |

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