feat: use concrete classes for hbase.client.registry.impl (#4043)

[igorbernstein2]

Previously we used runtime bytecode generation which prevented endusers from using hbase-site.xml to target bigtable in hbase 2 applications. This PR will create a concrete implementation for both AsyncRegistry for (hbase < 2.3) and ConnectionRegistry (for hbase >= 2.3).
To accomplish this, we use a local copy of the AsyncRegistry interface from HBase 2.2 that we use as a compile time target and then strip it from the resulting jar.

After this change end users should be able to use async apis in hbase 2 using hbase-site.xml. Examples:

<configuration>
    <property>
        <name>hbase.client.connection.impl</name>
        <value>com.google.cloud.bigtable.hbase2_x.BigtableConnection</value>
    </property>
    <property>
        <name>hbase.client.async.connection.impl</name>
        <value>org.apache.hadoop.hbase.client.BigtableAsyncConnection</value>
    </property>
    <property>
        <name>hbase.client.registry.impl</name>
        <!-- When using hbase >= 2.3, use this -->
        <value>org.apache.hadoop.hbase.client.BigtableConnectionRegistry</value>
        <!-- When using hbase < 2.3, use this -->
        <!-- <value>org.apache.hadoop.hbase.client.BigtableAsyncRegistry</value> -->
    </property>
    <property>
        <name>google.bigtable.project.id</name>
        <value>my-project</value>
    </property>
    <property>
        <name>google.bigtable.instance.id</name>
        <value>my-instance</value>
    </property>
    <property>
        <name>google.bigtable.app_profile.id</name>
        <value>my-app-profile</value>
    </property>
</configuration>

AsyncConnection asyncConnection = ConnectionFactory.createAsyncConnection().get();

[rkaregar]

Do we need any new tests to make sure everything works (e.g., passing the new BigtableConnectionRegistry class or using Async APIs) or they are already covered by the existing tests?

[igorbernstein2]

I plan on adding cross version tests, but that will be a much bigger change. I will add them soon