sp-core: `full_crypto` doesn't imply `std` (#11006)

* sp-core: `full_crypto` doesn't imply `std`

This pr changes the feature set of `secp256k1` to not use `global-context` when only the
`full_crypto` is enabled. It will be slower when the `std` feature is not enabled as the context
always needs to be recreated, but that is fine.

* Update client/cli/src/arg_enums.rs

Co-authored-by: Davide Galassi <[email protected]>

Co-authored-by: Davide Galassi <[email protected]>