Fix `epic_key` column in list call. Closes #75

[karanpopat]

Example query results

Results

``` > select key, id, epic_key, project_key from jira_issue where key in ('SBT-1', 'SBT-2', 'SBT-3') +-------+-------+----------+-------------+ | key | id | epic_key | project_key | +-------+-------+----------+-------------+ | SBT-3 | 10052 | SBT-1 | SBT | | SBT-1 | 10050 | | SBT | | SBT-2 | 10051 | SBT-1 | SBT | +-------+-------+----------+-------------+ Time: 396ms. Rows fetched: 1. Hydrate calls: 0. > select key, id, epic_key, project_key from jira_issue where project_key = 'SBT' +-------+-------+----------+-------------+ | key | id | epic_key | project_key | +-------+-------+----------+-------------+ | SBT-3 | 10052 | SBT-1 | SBT | | SBT-1 | 10050 | | SBT | | SBT-2 | 10051 | SBT-1 | SBT | +-------+-------+----------+-------------+ Time: 0.9s. Rows fetched: 3. Hydrate calls: 3. ```

[karanpopat]

@misraved Findings:

• The SDK is not returning the custom fields in the List call - epic_key is a part of the custom fields. Have raised an issue for the same - https://github.com/andygrunwald/go-jira/issues/636
• The issue persists even after updating the SDK
• One workaround could be by using the get api as hydrate for the column. Which is of course an extra api call.
• Another would be to use the raw searchIssue endpoint to fetch the List call data, which returns some means to fetch the epic data.

Have modified the list call in this PR to use the rest/api/2/search endpoint.