feat: add attachment download command

[admSla99]

Summary

Add `jira issue attachment download` command to download all attachments from a Jira issue.

Features

• Downloads all attachments from specified issue
• Default output directory: `.//`
• Custom output directory via `--output-dir` / `-o` flag
• Works with both Jira Cloud (v3 API) and Jira Server (v2 API)
• Human-readable file size display

Usage

  # Download all attachments to ./ISSUE-123/
  jira issue attachment download ISSUE-123

  # Download to custom directory
  jira issue attachment download ISSUE-123 -o ./downloads

  # Using alias
  jira issue att download ISSUE-123

Changes

• `pkg/jira/types.go` - Add `Attachment` type
• `pkg/jira/attachment.go` - Add `GetIssueAttachments` and `DownloadAttachment` methods
• `api/client.go` - Add `ProxyGetIssueAttachments` proxy function
• `internal/cmd/issue/attachment/` - Add attachment command with download subcommand
• Unit tests included

[admSla99]

Hello @ankitpokhrel can you or somebody else please review this ? This feature is realy needed for us in our company. Thank you

[saggl]

@admSla99 wauh really helpfull i also want this subcommand. @ankitpokhrel how long do you think need i wait?