Let's add `main_access_key` field to `keypair_list`.

## User story

- As a user, when I view the keypair list, I want to identify the main access key so that I can easily manage my keys.

## Context

The issue arises from the inability to identify the main access key in the keypair list. While users can query the main access key, this information is not available in the keypair section.

## Acceptance criteria

- The keypair list should clearly indicate which key is the main access key.
- Users should be able to easily distinguish the main access key from other keys in the list.

## Other information

N/A

JIRA Issue: BA-1734