Allow setting custom pragmas

[adri]

This allows for someone to set any PRAGMA keys and values, next to the most common ones. See the documentation for a full list: https://www2.sqlite.org/draft/pragma.html

One use-case is to set the cipher_compatibility PRAGMA which is supported by the SQLCipher extension:
https://www.zetetic.net/sqlcipher/sqlcipher-api/#cipher_compatibility

Fixes #227

[adri]

Before I switched to two separate functions I tried to...

Use a case statement, which was seen as one "indentation depth" too much by the linter.

Warning: 
┃ [F] → Function body is nested too deep (max depth is 2, was 3).
┃       lib/exqlite/connection.ex:384:11 #(Exqlite.Connection.set_custom_pragma)
[13](https://github.com/elixir-sqlite/exqlite/actions/runs/3862474027/jobs/6584066856#step:9:14)

I tried a with statement here, but that didn't work ;)

┃ [R] → `with` contains only one <- clause and an `else` branch, consider using `case` instead
┃       lib/exqlite/connection.ex:380 #(Exqlite.Connection.set_custom_pragmas)
[15](https://github.com/elixir-sqlite/exqlite/actions/runs/3862541622/jobs/6584174897#step:9:16)

So here we are with a separate function ;)

[warmwaffles]

Oh credo. We should just relax it instead. I prefer readability over pedantics.

[warmwaffles]

I assume it was complaining about this

with {:ok, list} <- Keyword.fetch(options, :custom_pragmas) do
  Enum.reduce_while(list, :ok, fn {key, value}, :ok ->
    case set_pragma(db, key, value) do
      :ok -> {:cont, :ok}
      {:error, _reason} -> {:halt, :error}
    end
  end)
else
  _otherwise -> :ok
end

[adri]

Yes that was it, and about a nested case statement :)

[warmwaffles]

I swear I'll get to this soon. I've got another bit of work for ecto_sqlite3 I need to knock out and then will refocus on this today.

[warmwaffles]

This looks great.

[adri]

Thank you so much!