A more modular `mail-auth`?

[CathalMullan]

Hi, I'd like to use mail-auth (well, really mail-send) in a project, but noticed a few issues with some of the features/dependencies being hard-coded here.

Currently tokio, ring, hickory-resolver, serde, ... are all pulled in and can't be disabled, even with default-features = false.

I'm wondering if you would be open to exposing more features?

Related, I'd also like to see the RustCrypto backend revived if possible, since it looks slated to be removed in the next release. If not, I'd understand.

My thinking would be something along the lines of:

[features]
default = ["verify", "sign", "report", "ring", "serde"]

verify = ["..."]
sign = ["..."]
generate = ["..."]
report = ["..."]

ring = ["..."]
aws-lc-rs = ["..."]
rust-crypto = ["..."]

serde = ["..."]
rkyv = ["..."]