Add idempotent to register_scheduler_plugin client

[asford]

Updates client-side interface of register_scheduler_plugin to pass through idempotent flag.

Extending #5545

Finalizing removal of kwargs support
deprecated in #5699
and removed in #6144

Closes #xxxx

• Tests added / passed
• Passes pre-commit run --all-files

[GPUtester]

Can one of the admins verify this patch?

[github-actions]

Unit Test Results

       16 files  +       1         16 suites  +1   7h 19m 26s ⏱️ + 28m 15s
  2 771 tests ±       0    2 692 ✔️ ±       0       78 💤 ±  0  1 ❌ ±0 
22 130 runs  +1 577  21 108 ✔️ +1 509  1 021 💤 +69  1 ❌  - 1 

For more details on these failures, see this check.

Results for commit 36aa7f3. ± Comparison against base commit 1937be7.

[mrocklin]

Thanks @asford !