allow using verify-aws flag for x509-parser dependency

[cfal]

x509-parser supports the verify-aws flag but the verify feature using ring is hardcoded:

https://github.com/rusticata/x509-parser/blob/master/Cargo.toml#L41
https://github.com/rustls/rcgen/blob/main/rcgen/Cargo.toml#L27

is it possible to specify the verify-aws flag so that aws-lc-rs users don't pull in ring when using rcgen with the aws_lc_rs feature?

[djc]

Good suggestion -- see the PR:

• Forward selected crypto backend to x509-parser #398