feat: add mtls support for NetHttpTransport

[arithmetic1728]

go/java-apiary-client-mtls (section 2.2.2.1)

Support
(1) passing keystore to SslContext constructor for client certificate and private key.
(2) creating keystore from certAndKey input stream.

Summary of new functions:

// In google-http-client/src/main/java/com/google/api/client/util/SecurityUtils.java
// create mtls key store from client cert and key input stream
public static KeyStore createMtlsKeyStore(InputStream certAndKey)

// In google-http-client/src/main/java/com/google/api/client/util/SslUtils.java
// init sslcontext with trust store and mtls key store
public static SSLContext initSslContext(
      SSLContext sslContext,
      KeyStore trustStore,
      TrustManagerFactory trustManagerFactory,
      KeyStore mtlsKeyStore,
      String mtlsKeystorePassword,
      KeyManagerFactory keyManagerFactory)

// In google-http-client/src/main/java/com/google/api/client/http/javanet/NetHttpTransport.java
public Builder trustCertificates(
        KeyStore trustStore, KeyStore mtlsKeyStore, String mtlsKeystorePassword)

// Indict if transport is mtls.
public boolean isMtls() {
    return this.isMtls;
}
``

[chingor13]

We also need to add tests for this

[arithmetic1728]

We also need to add tests for this

Tests are added now.

[chingor13]

We're going to hold off on approving and merging this approach (especially the isMtls() accessor) until the whole design is complete and approved.

[chingor13]

Looking really close.

[elharo]

For open source projects like this one, it's very helpful to have designs and issues on Github rather than in corp.

[chanseokoh]

Looks like this is to enable mutual TLS for NetHttpTransport. How about #904 (ApacheHttpTransport)? Is it that #904 should eventually be fixed?

[chingor13]

MTLS support will also need to be implemented for the Apache transport as well.

[arithmetic1728]

For open source projects like this one, it's very helpful to have designs and issues on Github rather than in corp.

I added a summary of the new functions added, I think they are self explanatory. The design doc is for client libs, http client is just a building block.

[arithmetic1728]

Looks like this is to enable mutual TLS for NetHttpTransport. How about #904 (ApacheHttpTransport)? Is it that #904 should eventually be fixed?

yes, mTLS support will be added to apache transport.

[chingor13]

A few nits

[chingor13]

@arithmetic1728 Let's follow up with a PR to add @Beta annotations to the new APIs and @since 1.38 to the javadoc of the new APIs