feat: add count operation to collections

[r1tsuu]

Description

Copy of #5930 to main branch

Adds operation count that counts the documents that are satisfying the query. Basically find without extra overhead (doesn't run drizzle/mongoose find), only totalDocs.

Usage:

Local API

const result = await payload.count({
  collection: 'users',
  where: {
    role: {
      equals: 'admin',
    },
  },
})
const { totalDocs } = result

REST API

const result = await fetch(`/api/posts/count`).then((res) => res.json())
const { totalDocs } = result

GraphQL

query {
  countPosts(where: { title: { equals: "Hello World" } }) {
    totalDocs
  }
}

• I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• I have added tests that prove my fix is effective or that my feature works
• Existing test suite passes locally with my changes
• I have made corresponding changes to the documentation

[shuowu]

When this feature will be released?