Add 5 community plugins (LDAP auth, dynamic flags/values, multichoice, user control)

[defojeco]

Summary

Adding 5 plugins built while deploying CTFd for a college diploma project:

• ctfd-ldap-plugin — Active Directory authentication with AES-256 offline
  credential cache, login mode switcher (Domain/Local), and AD-group → team
  auto-assignment. No alternative LDAP plugin exists for CTFd.

• ctfd-dynamic-flag — per-user/per-team computed flags (anti-sharing) via
  whitelisted hashing schemes (sha256, hmac-sha256, base64). Explicitly avoids
  exec()-based formula evaluation for security reasons (documented in the README).

• ctfd-dynamic-values — per-participant dynamic variables (unique IP,
  port, token) embedded directly into challenge descriptions via {{name}}
  syntax, deterministically generated from user/team identity.

• ctfd-plugin-multichoice — multiple choice challenge type (single/multi
  select), answer shuffling, and partial scoring.

• ctfd-user-control-plugin — profile field locking (username/email/password),
  per-challenge attempt limiting, and an audit log viewable in the admin panel.

All plugins are MIT licensed, documented (English + Russian README), and
include screenshots where applicable.

Testing

Each plugin has been running in production for a college CTF deployment.
Tested on CTFd 3.7.x / 3.8.5.

Compatibility

ctfd-dynamic-flag and ctfd-dynamic-values are designed to interoperate
(flags can reference dynamic values), but each works standalone.