Serialise bytes as base64 strings instead of arrays.

[nigel-andrews]

Description

Instead of serialising an array of bytes, encode that array to a base64 string and serialise it as a string instead.

Testing

Added tests to check the output of the serialisation.

[codecov-commenter]

Codecov Report

Merging #1608 (3b18908) into main (5945dbf) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1608      +/-   ##
==========================================
+ Coverage   93.75%   93.80%   +0.04%     
==========================================
  Files         251      252       +1     
  Lines       46278    46419     +141     
==========================================
+ Hits        43388    43543     +155     
+ Misses       2890     2876      -14     

Impacted Files	Coverage Δ
src/schema/value.rs	67.39% <100.00%> (+5.86%)	⬆️
src/indexer/sorted_doc_id_multivalue_column.rs	95.00% <0.00%> (-0.59%)	⬇️
src/fastfield/multivalued/writer.rs	98.34% <0.00%> (-0.03%)	⬇️
src/indexer/merger.rs	98.82% <0.00%> (-0.01%)	⬇️
src/reader/mod.rs	93.63% <0.00%> (ø)
src/schema/mod.rs	100.00% <0.00%> (ø)
src/indexer/mod.rs	100.00% <0.00%> (ø)
src/schema/text_options.rs	100.00% <0.00%> (ø)
src/fastfield/serializer/mod.rs	100.00% <0.00%> (ø)
src/core/inverted_index_reader.rs	72.86% <0.00%> (ø)
... and 7 more

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[fmassot]

LGTM. (PR is the same as #1607 which was already approved).