Dynamic config conversion improvements

[dnr]

** Do not merge until we're using Go 1.24 **

What changed?

• Split implementation of "constrained default" settings from "plain default" settings. This is more code and the diff looks complex, but the individual paths are both simpler than the mixed version.
• Add conversion cache using a weak map (requires Go 1.24).
• Remove GlobalCachedTypedValue.
• Use "raw" values for subscription dispatch deduping to avoid unnecessary conversions there.
• Deep copy default values when using mapstructure, to avoid problems with merging over shared default values.

Why?

• Fixes Optimize dynamic config value conversion #6756
• Performance improvement for "plain default" settings (almost all of them)
• Performance improvement for settings with complex converters
• Remove footgun in defaults that aren't scalar values

How did you test it?

existing+new unit tests