(fixes #5193) Serialize BucketMetadata#hashType as String

[clairemcginty]

see #5193 for motivation/details

Note that this change should have no user-facing impact--all public BucketMetadata constructors still use type HashType, not String.

I did standardize on the amount of overloaded constructors per BucketMetadata... some implementations had specializations for Void secondary keys and some did not. In the interest of simplicity (and since for the most part, users should not be constructing these directly), I removed the specialized overloads for Void secondary keys.

[clairemcginty]

this class/signature is required for Guava's Suppliers::memoize to construct a Serializable memoized supplier: https://github.com/google/guava/blob/v32.1.2/guava/src/com/google/common/base/Suppliers.java#L112-L115

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8962386) 63.05% compared to head (db631c9) 63.05%.
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Additional details and impacted files

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##             main    #5194   +/-   ##
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  Coverage   63.05%   63.05%           
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  Files         300      300           
  Lines       10947    10947           
  Branches      740      740           
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  Hits         6903     6903           
  Misses       4044     4044           

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[RustedBones]

If I read this correctly, this only improves case where one of the read partition hashType is not supported. We'll read metadata but fail during comparison with a message saying there is a metadata mismatch.
However if all hashType are equal but not supported, we're still failing at a later stage, correct ?

[RustedBones]

Note: Hash function is only used on write. Reader can work even if hash type is not known as long as they match

[clairemcginty]

Note: Hash function is only used on write. Reader can work even if hash type is not known as long as they match

Yup, reader only needs to access HashFunction as something Comparable across other metadata.json files 👍 After that, it doesn't need to invoke it at all.