pkcs8PEMRepresenation for RSA private keys

This PR adds a pkcs8PEMRepresentation property to RSA public keys.

Checklist

[x] I've run tests to see all new and existing tests pass
[x] I've followed the code style of the rest of the project
[x] I've read the Contribution Guidelines
[x] I've updated the documentation if necessary

If you've made changes to `gyb` files

[ ] ~~I've run .script/generate_boilerplate_files_with_gyb and included updated generated files in a commit of this pull request~~

Motivation:

There are two widely-used formats for public and private keys. PKCS#1 defines the format for RSA keys, which begin with BEGIN RSA {PUBLIC/PRIVATE KEY}

Later, as additional key types were implemented, more general containers were introduced. X.509 introduces SubjectPublicKeyInfo (SPKI), which defines a general structure for containing any type of public key (RSA or otherwise). PKCS#8 does the same for private keys. privateKey.pemRepresentation gives the PKCS#1 representation, but there's no equivalent for getting the PCKS8 representation.

Our implementation of RSA keys included a way to get both the new and old format of public keys (via publicKey.pemRepresentation and publicKey.pkcs1PEMRepresentation), but we didn't have a way to get both the new and old formats of private keys.

This PR introduces _RSA.PrivateKey.pkcs8PEMRepresentation to fill that gap.

Modifications:

_RSA's backing private key types have gained a new pkcs8PEMRepresentation property.
I adjusted the package file to pass the CRYPTO_IN_SWIFTPM and CRYPTO_IN_SWIFTPM_FORCE_BUILD_API flags to the _CryptoExtras target as well.
I updated the implementation of _CryptoExtras to check for those flags, rather than checking for #if canImport(Security). This made it easier to test the RSA changes when building on macOS.

Note that the BoringSSL implementation of pkcs8PEMRepresentation includes a trailing newline. The naive implementation using Security.framework would not have that trailing newline, but for consistency between the two implementations, I have added it to both.

I'm open to instead stripping off the trailing new line on the BoringSSL implementation, but I was less sure of how to implement that.

Result:

RSA private keys now have pkcs8PEMRepresentation.

apple / swift-crypto