Add version 2 of the elytron-oidc-client model and schema

Before we make changes to the schema for a particular subsystem that has already been included in a released version of WildFly, we first need to add a new version of the schema.

This means that before we add the new use-realm-role-mappings attribute to the elytron-oidc-client subsystem, the first step is to add version 2 of the elytron-oidc-client subsystem schema.

The main steps to do this are as follows:

1. In ElytronOidcExtension, add VERSION_2_0_0 and update CURRENT_PARSER, CURRENT_MODEL_VERSION, and initializeParsers accordingly
2. Add new ElytronOidcSubsystemParser_2_0 that extends ElytronOidcSubsystemParser_1_0. Initially, this class will only contain the updated NAMESPACE.
3. Add wildfly-elytron-oidc-client_2_0.xsd by initially copying from{{elytron-oidc-client_1_0.xsd}}
4. Add org.wildfly.extension.elytron.oidc.Subsystem_2_0_ParsingTestCase
5. Update the test xml files to reference urn:wildfly:elytron-oidc-client:2.0 instead of urn:wildfly:elytron-oidc-client:1.0

There are also some examples of how to add a new version here:

https://github.com/wildfly/wildfly/pull/13957
https://github.com/wildfly/wildfly-core/pull/4864/commits/591523addba0e1d6a3aa16b57c8cbad2bace4810

Once version 2 of the elytron-oidc-client schema has been added, we can add the new use-realm-role-mappings attribute to that version. The main thing to note is that we'll want to update wildfly-elytron-oidc-client_2_0.xsd to add the new attribute there. Similarly, we'll want to update ElytronOidcSubsystemParser_2_0 to override getSecureDeploymentParser to return the new secureDeploymentParser value with the new attribute added instead of updating ElytronOidcSubsystemParser_1_0.