Additional system tests for testing shim compatibility with the classic
client interface.

Also fixes a bug where encountering a retriable error after another
retriable error mid-stream raised an exception instead of retrying.

Fixes #1156

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Changes:

1. Moved the admin library from
`google.cloud.bigtable_admin/bigtable_admin_v2` to
`google.cloud.bigtable.admin`. The autogenerated portion of the library
was generated as follows:
```
py_gapic_library(
    name = "bigtable_admin_py_gapic",
    srcs = [":admin_proto"],
    grpc_service_config = "bigtableadmin_grpc_service_config.json",
    opt_args = [
        "python-gapic-namespace=google.cloud.bigtable",
        "python-gapic-name=admin",
    ],
    rest_numeric_enums = True,
    service_yaml = "bigtableadmin_v2.yaml",
    transport = "grpc+rest",
    deps = [
        "//google/iam/v1:iam_policy_py_proto",
    ],
)
```
This change will be committed close to when we will release the v3
client.

2. Created import aliasing to make sure that old imports from
`bigtable_admin/bigtable_admin_v2` still work as intended.
1. Created unit tests for the import aliasing. Since there's an existing
layer of import aliasing for objects within the GAPIC library, we create
unit tests to make sure the composition of existing and new import
aliasing works as intended.
1. Made the classic client use the new selective GAPIC
`BigtableTableAdminClient` instead of the `BaseBigtableTableAdminClient`
as its table admin client.

Fixes #1155

Changes:
- Added a new property `new_table_data_client` (might need to be
renamed) in the classic client for the new table data client
- Added two hidden kwargs (`client_info` and
`disable_background_channel_refresh`) to the new data client constructor
to support the classic client.

Fixes #1157