This is a first step to make it accept configuration. Most of the
changes are quite trivial, but I also ran into some difficulties with
query columns and filters. They need the schema for parsing, but parsing
happens before dependencies are instantiated (and even if it was not the
case, parsing is stateless). Therefore, I have added a `Validate()`
method that must be called after instantiation. Various bits `panic()`
if not validated to ensure we catch all cases.
The alternative to make the component manages a global state would have
been simpler but it would break once we add the ability to add or
disable columns.
This is a bit less type-safe. We could keep type safety by redefining
all the consts in `query_consts.go` in `common/schema`, but this is
pointless as the goal is to have arbitrary dimensions at some point.
We introduce an leaky abstraction for flows schema and use it for
migrations as a first step.
For views and dictionaries, we stop relying on a hash to know if they
need to be recreated, but we compare the select statements with our
target statement. This is a bit fragile, but strictly better than the
hash.
For data tables, we add the missing columns.
We give up on the abstraction of a migration step and just rely on
helper functions to get the same result. The migration code is now
shorter and we don't need to update it when adding new columns.
This is a preparatory work for #211 to allow a user to specify
additional fields to collect.
