Add support for Elixir Durations

[greg-rychlewski]

Addresses #688 besides the deprecation of Postgrex.Interval.

The following changes were made:

• allow encoding of Duration structs for postgres interval extension
• allow decoding postgres interval types to Duration structs if the option :interval_decode_type is set to Duration
• both of the above two are only allowed if the elixir version is at least 1.17.0

Some notes:

• when breaking down the postgres response (microseconds, days and months) into the fields for the Duration struct, I went with the same style we use for Postgrex.Interval: translate the lower units into the higher units as much as possible
• the type modifier postgres returns for intervals is not an integer from 0 to 6. i will try to find out what it means but it's not well documented so probably have to go through their source code. for now i always return precision of 6 because they are giving us microseconds in the response
• from what i can see we are not documenting anywhere the options that can be passed to any of our extensions. in a follow-up PR i'll try to collect them all and put them in a good place

[greg-rychlewski]

Would you want to add a :duration type to ecto?

[josevalim]

We should! :)