feat(server): reearthx to reearth accounts

[hexaforce]

Overview

Add the basic implementation needed before migrating from ReearthX to reearth-accounts.

What I've done

1. As part of switching from the reearthX package to reearth-accounts, I first added the necessary use cases, repositories, and other components while keeping the existing reearthX logic unchanged.

At this stage, calls are made to reearth-accounts, but the data referenced and the logic itself still follow the original reearthX implementation.

The major additions are based on the following corresponding logic:

reearthX	reearth-accounts
Operator	AccountsOperator
Workspace	AccountsWorkspace
User	AccountsUser

2. Currently, reearth-accounts is pointing to this branch:

Compared to reearthx, it simply copies the missing parts from internal into pkg.
https://github.com/reearth/reearth-accounts/tree/feat/reearthx-to-reearth-accounts

What I haven't done

How I tested

Which point I want you to review particularly

Memo

Checklist

• Verified backward compatibility related to feature modifications (if not compatible, reported deployment notes to the next release owner).
• Confirmed backward compatibility for migrations.
• Verified that no personally identifiable information (PII) is included in any values that may be displayed.