feat: add InjectEntityManager decorator for MikroORM and upgrade document

[ghostoy]

Checklist

• npm test passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

mikro

Description of change

由于MikroORM从5.7开始，开始废弃Repository上的persist, flush等接口，v6计划彻底删除这些接口，导致要保存到数据库的时候要这样写：

class BookService {
  @InjectRepository(Book)
  bookRepository: EntityRepository<Book>;

  async create() {
    const book = new Book(/** 初始化 **/);
    await this.bookRepository.getEntityManager().persistAndFlush(book);
  }
}

增加InjectEntityManager直接注入EntityManager后，写法如下：

class BookService {
  @InjectEntityManager()
  em: EntityManager;

  async create() {
    const book = new Book(/** 初始化 **/);
    await this.em.persistAndFlush(book);
  }
}

[codecov-commenter]

Codecov Report

Patch coverage: 91.66% and project coverage change: -0.14% ⚠️

Comparison is base (2d93592) 81.78% compared to head (ffe0c82) 81.64%.

❗ Current head ffe0c82 differs from pull request most recent head 069754e. Consider uploading reports for the commit 069754e to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3114      +/-   ##
==========================================
- Coverage   81.78%   81.64%   -0.14%     
==========================================
  Files         437      437              
  Lines       15918    15929      +11     
  Branches     3769     3773       +4     
==========================================
- Hits        13018    13005      -13     
- Misses       2631     2650      +19     
- Partials      269      274       +5     

Files Changed	Coverage Δ
packages/mikro/src/configuration.ts	90.69% <88.88%> (-0.74%)	⬇️
packages/mikro/src/decorator.ts	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes

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