[api_summary] Track parameter default values

[kevmoo]

Currently, api_summary tracks parameter names and types for methods/functions, but does not track parameter default values (e.g. void f([int x = 1])). Modifying a default value (e.g. from 1 to 2) is a silent behavioral breaking change for consumers relying on the previous defaults, but api_summary does not generate a diff.

Example

Changing:

void f([int x = 1]) {}

to:

void f([int x = 2]) {}

currently produces identical api_summary output.

[lrhn]

May or may not be breaking.
Default values are part of the implementation. If you can get the same behavior with a different default value, users don't need to know. That's usually the sign that it is so: That users don't need to know the default value.

If the default value is a sentinel, then it doesn't matter what it is, as long as the user can't provide it.
If the default value is one of a number of equivalent values (say something like .padLeft which treats all numbers <= 0 the same, doesn't matter if the default is 0 or -1), then it doesn't matter that it changes.
Wont catch a nullable parameter and a function body starting with param ??= defaultValue; and changing that default value.

Its still a signal, but not necessarily a break.