fix: remove cache and database adaptor flavor enums

[wschurman]

Why

Follow-up on #108 (comment) and an inline comment indicating that this would eventually just become a string.

This pattern of using a hardcoded enum wasn't flexible enough to support additional third-party/private adaptor implementations and/or multiple adaptors in the same application.

How

Change it to a string and add definition validation to ensure definition is supplied for requested flavor.

Test Plan

Run all tests.

[codecov]

Codecov Report

Merging #109 (ab5ff4c) into master (b7309e1) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #109   +/-   ##
=======================================
  Coverage   94.96%   94.96%           
=======================================
  Files          66       66           
  Lines        1627     1627           
  Branches      195      193    -2     
=======================================
  Hits         1545     1545           
  Misses         80       80           
  Partials        2        2           

Flag	Coverage Δ
integration	94.96% <100.00%> (ø)
unittest	94.96% <100.00%> (ø)

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Impacted Files	Coverage Δ
packages/entity-example/src/data.ts	85.71% <ø> (ø)
packages/entity-example/src/entities/NoteEntity.ts	100.00% <ø> (ø)
.../src/__integration-tests__/entities/ChildEntity.ts	100.00% <ø> (ø)
...src/__integration-tests__/entities/ParentEntity.ts	100.00% <ø> (ø)
packages/entity/src/EntityCompanionProvider.ts	100.00% <100.00%> (ø)
...s/testing/createUnitTestEntityCompanionProvider.ts	100.00% <100.00%> (ø)

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