feat(extensions): add `x-no-omitempty`

[jamietanna]

In adding x-omitzero in #1944, it's likely not an issue (via) when both omitzero and omitempty are used:

If both "omitempty" and "omitzero" are specified, the field will be omitted if the value is either empty or zero (or both).

However, there may be cases that we want to only use omitzero, so we should use a new extension,x-no-omitempty, accordingly.

I don't have any meaningful usecases that this would be useful to add for - any feedback would be appreciated with helping work out what makes sense here!

[zoop-btc]

I don't like pointers and never use them. So I always have all fields optional with x-go-type-skip-optional-pointer: true. Upgrading to go 24+ broke this. So I'd like to have this feature.