Add support for configuring PreparedStatementCache

[vijayakumarsuraj]

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Issue description

https://github.com/r2dbc/r2dbc-mssql/issues/227 - Allow configuring the prepared statement cache

New Public APIs

• PreparedStatementCache is now public.
• Added discovery option preparedStatementCache.

Additional context

The preparedStatementCache discovery option behaves similar to the one in r2dbc-postgresql.

• -1 = indefinite cache
• 0 = no cache
• n = LRU cache with max size n.
• Additionally, it also accepts the class name of a PreparedStatementCache implementation.

[mp911de]

Thanks a lot. For a proper implementation of the cache we need to call sp_unprepare <handle> somewhere, ideally when the cache entry is being removed. Calling unprepare can interfere with the ongoing query so that might require some special attention.

[xnart]

For now, can we provide only NoPreparedStatementCache to allow disabling cache ? If it is ok i can create new PR.