GCU support for Cisco-8000 features

[rbpittman]

What I did

Fill in several "Unknown" fields in the table from PR #2791 for Cisco-8000.

How I did it

How to verify it

Verified test pass for test_pg_headroom_update.py

Previous command output (if the output of a command-line utility has changed)

When trying to apply a patch to update the xoff field for a lossless buffer profile:

$ config apply-patch /home/cisco/patch.json
Patch Applier: Patch application starting.
Patch Applier: Patch: [{"value": "440600", "op": "replace", "path": "/BUFFER_PROFILE/pg_lossless_100000_5m_profile/xoff"}]
Patch Applier: Getting current config db.
Patch Applier: Simulating the target full config after applying the patch.
Patch Applier: Validating all JsonPatch operations are permitted on the specified fields
Failed to apply patch
Usage: config apply-patch [OPTIONS] PATCH_FILE_PATH
Try "config apply-patch -h" for help.

Error: Modification of BUFFER_PROFILE table is illegal- validating function generic_config_updater.field_operation_validators.rdma_config_update_validator returned False

New command output (if the output of a command-line utility has changed)

$ config apply-patch /home/cisco/patch.json
Patch Applier: Patch application starting.
Patch Applier: Patch: [{"value": "440600", "op": "replace", "path": "/BUFFER_PROFILE/pg_lossless_100000_5m_profile/xoff"}]
Patch Applier: Getting current config db.
Patch Applier: Simulating the target full config after applying the patch.
Patch Applier: Validating all JsonPatch operations are permitted on the specified fields
Patch Applier: Validating target config does not have empty tables, since they do not show up in ConfigDb.
Patch Applier: Sorting patch updates.
Patch Applier: The patch was sorted into 1 change:
Patch Applier:   * [{"op": "replace", "path": "/BUFFER_PROFILE/pg_lossless_100000_5m_profile/xoff", "value": "440600"}]
Patch Applier: Applying 1 change in order:
Patch Applier:   * [{"op": "replace", "path": "/BUFFER_PROFILE/pg_lossless_100000_5m_profile/xoff", "value": "440600"}]
Patch Applier: Verifying patch updates are reflected on ConfigDB.
Patch Applier: Patch application completed.
Patch applied successfully.

[wsycqyz]

LGTM