remove user and password from DSN record

[akdor1154]

Hey, This PR removes auth stuff from the DSN record of the Connection struct. Instead it is passed in at connection time them forgotten. Two rationales:

• the password is sensitive information and it's really easy at the moment to have it printed by mistake to e.g. notebook output.
• allows to standardize the (dsn; user, pass) method to work on both DSN names and raw connection strings

Additionally, it allows me to add an extraauth parameter which is appended as-is and likewise treated as sensitive info. This will be database specific, and could be used for e.g. auth tokens, Snowflake warehouse specification, etc etc, basically things for where my code's connection string needs to be the same per-user, but users may still need to put account-specific config to get a DB connection. (This model actually follows how PowerBI models ODBC connections, and I figure that they have done a bit more solution design across different DBs than I have :) )

Added public API:

function Connection(dsn::AbstractString; user=nothing, password=nothing, extraauth=nothing)

Tests added and checked against MariaDB locally (we'll see what Travis says).. I will do further testing if Travis passes.

Any and all feedback welcome, this is my first public work in Julia so there are probably style issues at the very least.

[codecov[bot]]

Codecov Report

Merging #305 into master will increase coverage by 0.17%. The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #305      +/-   ##
==========================================
+ Coverage   78.24%   78.42%   +0.17%     
==========================================
  Files           6        6              
  Lines         754      760       +6     
==========================================
+ Hits          590      596       +6     
  Misses        164      164              

Impacted Files	Coverage Δ
src/dbinterface.jl	93.67% <87.50%> (+0.22%)	:arrow_up:
src/API.jl	67.35% <100.00%> (ø)

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[akdor1154]

thanks for the response, yep I had finished at that point but will fix up your review points in a little bit.

I am a bit confused by

I don't see the user/password fields removed from the Connection struct yet?

Have I missed/misunderstood something? I preserved

Connection(dsn::AbstractString, usr, pwd) = Connection(dsn; user=usr, password=pwd)

to not break your API but as far as i can tell user and password never actually end up in the Connection struct (they are used to create the dbc from API.Connect/SQLDriverConnect I guess)