openpgp-card-sequoia: Allow disabling default crypto backend.

Previously the list of features of `sequoia-openpgp` unconditionally
included `crypto-nettle`. That was preventing the use of this crate in
pure Rust setups.

This commit allows disabling crypto backends and it is up to the
crate user to explicitly include `sequoia_openpgp` features that they
want.

See: https://gitlab.com/sequoia-pgp/sequoia/-/blob/main/README.md#note

openpgp-card-sequoia/Cargo.toml

@@ -12,7 +12,7 @@ repository = "https://gitlab.com/openpgp-card/openpgp-card"
 documentation = "https://docs.rs/crate/openpgp-card-sequoia"
 
 [dependencies]
-sequoia-openpgp = "1.4"
+sequoia-openpgp = { version = "1.4", default-features = false }
 openpgp-card = { path = "../openpgp-card", version = "0.3.3" }
 chrono = "0.4"
 anyhow = "1"
@@ -26,5 +26,8 @@ openpgp-card-pcsc = { path = "../pcsc", version = "0.3" }
 env_logger = "0.9"
 testresult = "0.3.0"
 
+[features]
+default = ["sequoia-openpgp/default"]
+
 [[example]]
 name = "test"