Add support for rustls

[bswinnerton]

This is the final pull request in a series to add support for using rustls as the underlying TLS library for bgpkit-parser's dependencies. The two other related pull requests were:

• Add feature to support rustls-based HTTPS requests oneio#21
• Add feature to support rustls-based HTTPS requests bgpkit-commons#2 (not a dependency, but similar in nature)

This pull request introduces a new feature flag known as parser-rustls that enables oneio's rustls feature flag. I'm not sure this is the best option, and would love feedback on this approach. Alternatively, we can also explore adding two new feature flags: native-tls and rustls, similar to what was done in bgpkit/oneio#21, but that feels like more of a breaking change, and this is a far more popular crate and would require users to explicitly opt into both flags when they want to use parser: features = ["parser", "native-tls"] or features = ["parser", "rustls"].

Happy to make updates based on the preferred approach, and thank you for considering this PR!

[bswinnerton]

Ah, actually on second thought the parser-rustls based approach won't work without further changes:

bgpkit-parser/src/lib.rs

Lines 375 to 376 in 0554fff

	#[cfg(feature = "parser")]
	pub mod parser;

I'll explore a few other options.

[bswinnerton]

Alright, I've switched the strategy around a bit and introduced two new feature flags: native-tls and rustls, which must be passed alongside parser.

Would love feedback on this approach as opposed to the one written in the OP.

[digizeph]

Just released oneio=0.13.1 that adds lib-native-tls and lib-rustls flags.
https://github.com/bgpkit/oneio/releases/tag/v0.13.1

This should make the dependencies cleaner.

[bswinnerton]

Looks great! Thanks @digizeph.