Replace most Optional<T&> with T*.

[BillyONeal]

I did this mostly to see if copilot-cli could do it, and it was able to do most of it. There's no 'business case' to merge it and it will create merge conflicts. But it is a style change I've wanted to do for ages and just never found the time to do; the changes seemed 'local' enough that LLMs may be able to do it so that's what I tried. Would be totally happy to throw this away as a result.

Note that src/vcpkg/commands.check-support.cpp was previously copying the PlatformExpression::Context when the behavior everywhere else in vcpkg is to use the returned context directly, so that is a behavior change.

Remaining Optional<T&> uses:

• Tests for Optional<T&> itself
• Tests for adapt_context_to_expected's handling of Optional<T&> (in particular, that that needs to be treated like an lvalue even though a returned Optional<T&> is usually a prvalue)
• InstallPlanAction::source_control_file_and_location removed in Remove optionality in ActionPlan and Summary #1894