Refactor + Fix for making table headers (#45)

Issue:
In the query response, not all keys may be present, leading to columns
in the displayed table having mismatching headers, as these headers are
derived from the keys of the first record.

Fix:
Determining headers on two scenarios:

1. When the query includes '*', indicating the selection of all columns,
headers are generated from the schema by mapping parseable data types to
Grafana's own data types. Any gaps are filled by iterating over the
generated data frame.

2. If the query selectively chooses columns, headers are created from
the first record, assuming that the SQL query results fill any gaps.

Author: balaji-jr