Use rustls for interop tests (#125)

* Use rustls for interop tests

This commit changes the interop tests to use rustls instead of openssl.
Apparently in the past there was some issue with this, but it seems to
work OK to me.

* Use certificates with larger key sizes for interop

This commit switches out the certificates used for testing interop to be
based on 4096-bit RSA keys, allowing rustls to be used for the interop
testing instead of OpenSSL.

The keys are generated using Terraform, although the state file is not
committed. A README.md is added to the data directory that explains how
to use Terraform to rotate the test certificates if this is ever
desirable.

This is desirable in order that none of the crates which `cargo build
--all` will build have the `openssl` feature, which should allow Tonic
to build on Windows with no issues.

tonic-interop/data/server1.pem

@@ -1,16 +1,20 @@
 -----BEGIN CERTIFICATE-----
-MIICnDCCAgWgAwIBAgIBBzANBgkqhkiG9w0BAQsFADBWMQswCQYDVQQGEwJBVTET
-MBEGA1UECBMKU29tZS1TdGF0ZTEhMB8GA1UEChMYSW50ZXJuZXQgV2lkZ2l0cyBQ
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 -----END CERTIFICATE-----