Change the attribute dictionary syntax to separate name and value with '='.

The current syntax separates the name and value with ':', but ':' is already overloaded by several other things(e.g. trailing types). This makes the syntax difficult to parse in some situtations:

Old:
  "foo: 10 : i32"

New:
  "foo = 10 : i32"
PiperOrigin-RevId: 255097928