fix: keep _metadata when serializing to dict and json

[JohannesMessner]

This bug is the root cause of multi-modal documents not working after being (de)serilaized from/to json or dict.
Details here: #429

Better multi-modal support (#425) partially depends on this.

There seems to be no super elegant way of solving this with pydantic, based on this and this.

I see two options:

1. make metadata public, so rename _metadata -> metadata
2. set and validate _metadata manually

In order not to change the pydantic model of Document, this PR implements option 2.

[codecov]

Codecov Report

Merging #443 (615f3ce) into main (b8ba0f8) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #443      +/-   ##
==========================================
- Coverage   86.62%   86.61%   -0.02%     
==========================================
  Files         134      134              
  Lines        6477     6486       +9     
==========================================
+ Hits         5611     5618       +7     
- Misses        866      868       +2     

Flag	Coverage Δ
docarray	86.61% <100.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
docarray/__init__.py	75.00% <100.00%> (ø)
docarray/document/pydantic_model.py	98.03% <100.00%> (+0.42%)	⬆️
docarray/helper.py	81.90% <0.00%> (-0.91%)	⬇️

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