Add dynamic adaptation properties prototype

[tomalrussell]

Just creating this as a WIP pull request so we can discuss here.

[tomalrussell]

I had a quick look at options for indexing JSONB, and it looks like we could index on an expression [1], and a hash index might be a relatively small and fast option if we always wanted a simple equality check:

CREATE INDEX ON test USING HASH ((data ->> 'field'));

EXPLAIN SELECT * FROM test WHERE data->>'field' = 'value';

As well as the more flexible GIN approach [2], which is the main thing discussed in the docs and articles I found, e.g. [3].

1. https://www.postgresql.org/docs/current/indexes-expressional.html
2. https://www.postgresql.org/docs/current/datatype-json.html#JSON-INDEXING
3. https://pganalyze.com/blog/gin-index