fix: Allow cargo build --all-features

[jayvdb]

I believe there is also a crate which allows mutually exclusive features, so we could do that instead.

[jayvdb]

https://github.com/omid/mutually_exclusive_features could be used to enforce that one of time or chrono is enabled.

Currently it isnt possible to disable both features, as that results in build errors - many due to Timestamp not having a definition, but also things like missing members of JobContext that other code relies on.

error[E0425]: cannot find value `sleeper` in this scope
  --> packages/apalis-core/src/worker/ready.rs:82:21
   |
82 |                     sleeper.sleep(interval).await;
   |                     ^^^^^^^ not found in this scope

error[E0063]: missing field `run_at` in initializer of `JobContext`
  --> packages/apalis-core/src/context.rs:51:9
   |
51 |         JobContext {
   |         ^^^^^^^^^^ missing `run_at`

[jayvdb]

Probably related, each Cargo.toml includes

[package.metadata.docs.rs]
# defines the configuration attribute `docsrs`
rustdoc-args = ["--cfg", "docsrs"]
all-features = true

I guess that means docs.rs is going to wants all features to be able to compile together.
https://docs.rs/about/metadata

so if we dont go ahead with this PR, then we need to remove all-features = true and instead manually list the features which should be used on docs.rs