Log correct audit event when changing custom role

What does this MR do and why?

It changes the format of audit event details around group/project membership changes.

When handling a default role (static role) we use Default role: ROLE NAME label. When handling a custom role we use Custom role: ROLE NAME label.

A new method was added to the Gitlab::Access as I expect it to be used on more places in the future.

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Before	After

How to set up and validate locally

If you don't have any custom roles stored on your instance

1. Run the following graphQL query to create a custom role (skip groupPath if you are on self-managed instance) (https://gdk.test:3443/-/graphql-explorer)

mutation {
  memberRoleCreate(input: {
    groupPath: "flightjs"
    name: "Role name", 
    description: "This is a new test role",
    baseAccessLevel: GUEST,
    permissions: [READ_CODE, READ_VULNERABILITY]
  }) {
    errors
    memberRole {
      id
      name
    }
  }
}

1. Visit a group or project member page such as https://gdk.test:3443/groups/flightjs/-/group_members
2. Assign this custom role to a member
3. Assign a static role to a member with a custom role
4. Visit Admin - Monitoring - Audit logs or Group -> Secure -> Admin events and verify the correct messages are displayed (the name of the custom role should be displayed)

Related to #427954 (closed)