Make sea-orm-cli DB backends optional

[Expurple]

The issue

My project uses a local sea-orm-cli (#1889) and only connects to PostgreSQL. Cargo.toml has the following:

[dependencies]
sea-orm = { version = "0.12.15", features = [
  "debug-print",
  "macros",
  "postgres-array",
  "runtime-tokio-rustls",
  "sqlx-postgres",
] }
sea-orm-cli = { version = "=0.12.15", default-features = false, features = [
  "codegen",
  "cli",
  "runtime-tokio-rustls",
] }
sea-orm-migration = { version = "0.12.15", features = [
    "sqlx-postgres",
    "runtime-tokio-rustls",
    "with-chrono",
] }

If I comment out sea-orm-cli, then sea-orm and sea-orm-migration behave nicely and only pull in sqlx-postgres. However, sea-orm-cli pulls in sqlx-sqlite and sqlx-mysql. I'd like to avoid that.

Details

Unlike other SeaQL crates, sea-orm-cli doesn't have any feature flags to control this behavior. The problem seems to originate here, where it pulls in sea-schema/sqlx-all:

# sea-orm-cli/Cargo.toml
[features]
codegen = ["sea-schema/sqlx-all", "sea-orm-codegen"]

Why does it need that? Can this be avoided?

With some guidance, it's possible that I can contribute the missing flags myself.

[Expurple]

Reported again by another user: #2320

[xamgore]

-sea-orm = { version = "0.12.15", features = [
+sea-orm = { version = "=1.0.0", default-features = false, features = [