0.3.1 release preparation

[cpu]

Proposed release notes

• New rustls_platform_verifier::tls_config_with_provider() function for convenient construction of a rustls::ClientConfig configured with the platform verifier, and a specific rustls::crypto::CryptoProvider.
• New Verifier::with_provider() and Verifier::set_provider() fns for constructing or updating a Verifier with a specific rustls::crypto::CryptoProvider.
• Support for Apple tvOS.

Post-merge steps

• Generate Android Maven artifacts locally - N/A - no non-test changes made under android/
• Create and push Git tag - created v/0.3.1 tag.
• cargo publish for each required crate, based on release steps - Published rustls-platform-verifier v0.3.1 at registry crates-io
• Create companion GitHub release - created GitHub release

[cpu]

v0.3.1 published.