Add -gateway flag, deprecation warning for -broadcaster

[eliteprox]

What does this pull request do? Explain your changes. (required) Adds the new -gateway flag while maintaining support for -broadcaster with a deprecation warning on startup. This change does not address refactoring associated with variable names and types, which will be addressed in a later change.

Specific updates (required)

• Updates starter.go and livepeer.go to include the -gateway flag
• Adds deprecation warning to startup and cli help

How did you test each of these updates (required)

• Started a gateway node using the -gateway flag
• Started a gateway node using the -broadcaster flag, noted deprecation warning

W0508 01:33:14.643250  158728 starter.go:636] -broadcaster flag is deprecated and will be removed in a future release. Please use -gateway instead

Does this pull request close any open issues?

LIV-287 Allow users to use -gateway instead of -broadcaster

Checklist:

• [x] Read the contribution guide
• [x] make runs successfully
• [x] All tests in ./test.sh pass
• [] README and other documentation updated
• [ ] Pending changelog updated

[rickstaa]

@eliteprox great! Merged into the upstream 🚀.

[rickstaa]

@hthillman We've decided to deprecate the Broadcaster term in the AI Subnet as it no longer fits well. Do you think it's beneficial to merge this pull request into the main branch now, or is it too early?