Parallelize flow compilation on submission path

[DaisyModi]

Dear Gobblin maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses the following Gobblin JIRA issues and references them in the PR title.

Description

• Here are some details about my PR:

Every flow gets compiled twice — once on submission and once on execution. The execution path already runs on a thread pool of size 3 (DagProcessingEngine), proving that compileFlow() is thread-safe. However, the submission path was fully serialized by:

1. A synchronized block on SpecCatalogListenersList.onAddSpec()
2. A single-thread executor inside CallbacksDispatcher

This caused unnecessary queuing when multiple flows were submitted simultaneously.

Changes:

• Removed synchronized from SpecCatalogListenersList.onAddSpec() to allow concurrent flow compilations
• Added a new constructor to SpecCatalogListenersList that accepts an ExecutorService
• Updated FlowCatalog to create a configurable FixedThreadPool (default: 3 threads) via gobblin.service.specCatalogListener.numThreads

Safety:

• onDeleteSpec/onUpdateSpec remain synchronized (topology changes are infrequent)
• addListener/removeListener remain synchronized
• topologySpecMap is a ConcurrentMap — safe for concurrent reads
• The execution path (DagProcessingEngine with 3 threads) already validates compileFlow() thread-safety

Tests

• My PR does not need new testing because it only removes serialization that was already proven unnecessary by the execution path (DagProcessingEngine) which runs compileFlow() from 3 concurrent threads. Existing FlowCatalogTest and MultiHopFlowCompilerTest cover the compilation flow.

Commits

• My commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"