[Rust] Allow to supply a default string value

[kistz]

Description of Changes

Rust modules cannot do it at the moment.
Allow it by taking in a &'static str.
The approach in the macro is safe because we always have a std::String enforeced by the SpacetimeType implementation.

API and ABI breaking changes

None. Additive changes

TODO: Update the documentation aswell if approach is good enough.

Expected complexity level and risk

1

Testing

• Migrated a test module locally and it worked
• Added a test for it

[kistz]

@gefjon could you take a look at this? :>
if the approach is sufficiently good i would adjust the docs 😇

[gefjon]

This seems totally reasonable, though it will need updates to be compatible with #5630 , and to use the tests in #5618 . I am unsure whether we'll merge this PR or that chain first.

[bfops]

Regardless of the order of PR merges, I think the tests in #5618 are the best coverage for this, so I had the bot revert the test changes. Other than that, I'm fine to merge in any order if you want to move this out of draft.

[kistz]

@bfops @gefjon cool ty :>
i adjsuted the docs (removed rust disclaimer)

moved out of draft and ready to merge from my side :>

Have a good one guys!

( if i am required to do anything else np just tell me 👍 )