Split config discovery and configuration logic

[ubombi]

Make configuration pluggable and implement:

• Basic mandatory schema discovery

Optional:

• Autodiscovery based on schema, PKs, FKs
• Conftags based config
• External config

Each config applies changes on top of previous one in same order as passed to client.

E.G:

posgtres.New(
    discovery.WithSchema(),               // parse existing PKs, FKs, Indecies. Constraints 
    discovery.WithConftags("prefix_"),    // Apply conftags changes   E.G: skip some fields
    discovery.WithFileconfig(yamlfile),   // Apply additional external config, E.G: change index name
)