Add `engagement_type` column to jobs table

[yawboakye]

Engagement type can have one of the following values: full-time, part-time, gig.

[Robbieakoto]

@yawboakye @claeusdev @claudey I realised that we already have a duration column which holds these same values in this issue description. Kindly check that

[yawboakye]

@Robbieakoto Good call out. Currently, the duration property of a job post is overloaded. We want to simplify it to a non-negative integer value that represents the number of months that the job is available for. A full-time job can be available for 3 months. Similarly, a part-time job can be available for 12 months, or indefinitely. We want to use the new engagement_type property to capture the part-time or full-time-ness of the job, irrespective of duration.

[Robbieakoto]

@yawboakye Alright that's clear. Would go with this then.