Add "keepalive" flag to CLI to enable loaded extensions to control process lifetime

[Dewb]

Purpose

This PR adds a -k/--keepalive flag to the CLI. This will launch the engine and wait for additional commands to be queued from loaded extensions; the process will not exit until a loaded extension shuts down the model.

This adds a new boolean field to the public class StartupUtils.CommandLineArguments.

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• User facing strings, if any, are extracted into *.resx files
• All tests pass using the self-service CI.
• n/a Snapshot of UI changes, if any.
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.
• n/a This PR modifies some build requirements and the readme is updated

Reviewers

(FILL ME IN) Reviewer 1 (If possible, assign the Reviewer for the PR)

(FILL ME IN, optional) Any additional notes to reviewers or testers.

FYIs

(FILL ME IN, Optional) Names of anyone else you wish to be notified of

[sm6srw]

I can't see any logic in the test failure. Looks like a sporadic failure to me. Other than that: LGTM!

[aparajit-pratap]

@Dewb could you give some context to this change? I may have missed the discussion if it took place elsewhere.