chore(core-database-postgres): convert BYTEA to/from string

[vasild]

Engage pg-promise's ability to modify the values retrieved from the
database on the fly (the cb argument of
https://vitaly-t.github.io/pg-promise/Database.html#result).

This way we convert transparently the vendorField column which comes as
a Buffer from the database to an UTF8 string. In the other direction,
when inserting into the database, the string is converted to a Buffer
using the init property of the column definition.

Rename findMany and findManyWithCount to findAny and findAnyWithCount.
pg-promise has a method many() which throws if no rows are returned. It
also has a method any() which accepts 0, 1 or more rows as a result
(same as manyOrNone() or result()). Our findMany* methods do
not call the many() method but rather accept any number of rows as a
result.

Summary

Checklist

• Documentation (if necessary)
• Tests (if necessary)
• Ready to be merged

[codecov]

Codecov Report

Merging #3058 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #3058      +/-   ##
===========================================
+ Coverage    65.62%   65.62%   +<.01%     
===========================================
  Files          425      425              
  Lines        11733    11734       +1     
  Branches      1601     1602       +1     
===========================================
+ Hits          7700     7701       +1     
  Misses        3997     3997              
  Partials        36       36

Impacted Files	Coverage Δ
...s/core-database-postgres/src/models/transaction.ts	100% <100%> (ø)	⬆️

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[spkjp]

We don't need the to string conversion, so please remove it. One-way is sufficient since the column is technically only needed by core-api for searching.