Signal w/ Start API Improvements - Java

[drewhoskins-temporal]

Is your feature request related to a problem? Please describe.
BatchRequest is hard to use, i.e. it has some affordances that would be illegal.

• First item must be the workflow start
• Second must be signal
  We've never added support for other operations to it, such as Update, which means it might mislead people into thinking it could be used for an Update.

Describe the solution you'd like
Suggest deprecating it and replacing it with something

• easier to use
• more consistent with updateWithStart.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[drewhoskins-temporal]

Related: #915