This commit adds a simple API for specifying the TLS certificate a GRPC
client will present (via the same `Identity` wrapper as a server cert is
configured). It also adds an API to specify which CA certificate client
TLS certificates will be validated against for servers.
This commit reworks TLS configuration of both servers and endpoints in
order to provide a more flexible API. We now add options to configure
the selected TLS library using the appropriate 'native' configuration
structures, as well as retaining the existing simplier interface which
is compatible with both.
The new API can also be easily extended to support simple interfaces for
configuring mTLS and a range of other options without creating sprawl
in the builders for `Server` and `Endpoint`.
Currently, using an RSA key - or indeed any text which is not a
PEM-encoded PKCS8-format key with RusTLS panics with an index-out-of-
range exception.
This commit introduces two new types of `TlsError` which indicate
problems with loading a certificate or private key:
- `CertificateParseError`, indicating that the certificate cannot be
parsed
- `PrivateKeyParseError`, indicating that the private key cannot be
parsed.
Additionally, in the case that reading the private key as PKCS8 fails,
we now try loading it as an RSA key before failing.
