Add role column to `members list` output

[mmanciop]

Context

The dash0 -X members list command currently displays name, email, ID, and URL columns.
Users would like to see each member's role (admin, basic_member, etc.) in the output.

Blocked by

The members list API (GET /api/members) does not currently return the role in MemberDefinition.

See: dash0hq/dash0-api-client-go#5

Implementation

Once the API returns the role:

1. Extract the role from MemberDefinition in MemberValues() (internal/members/list.go).
2. Add a role column to MemberListDefaultColumns.
3. Add HEADER_ROLE constant to internal/constants.go.
4. Update integration test fixtures and CSV header assertions.
5. Update docs/commands.md with the new column in example output and column aliases.