feat: add output file for export-schema

[stillmatic]

Description

See #3032 for motivation

> strawberry export-schema backend.schema --output schema.gql
Schema exported to schema.gql
> strawberry export-schema backend.schema
# prints the result correctly

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Documentation

Issues Fixed or Closed by This PR

• Add output file to export-schema #3032

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[botberry]

Thanks for adding the RELEASE.md file!

Here's a preview of the changelog:

---

Adds a new flag to export-schema command, --output, which allows the user to specify the output file. If unset (current behavior), the command will continue to print to stdout.

---

Here's the preview release card for twitter:

Here's the tweet text:

🆕 Release (next) is out! Thanks to @hingeloss for the PR 👏

Get it here 👉 https://github.com/strawberry-graphql/strawberry/releases/tag/(next)

[botberry]

Hi, thanks for contributing to Strawberry 🍓!

We noticed that this PR is missing a RELEASE.md file. We use that to automatically do releases here on GitHub and, most importantly, to PyPI!

So as soon as this PR is merged, a release will be made 🚀.

Here's an example of RELEASE.md:

Release type: patch

Description of the changes, ideally with some examples, if adding a new feature.

Release type can be one of patch, minor or major. We use semver, so make sure to pick the appropriate type. If in doubt feel free to ask :)

Here's the tweet text:

🆕 Release (next) is out! Thanks to @hingeloss for the PR 👏

Get it here 👉 https://github.com/strawberry-graphql/strawberry/releases/tag/(next)

[codecov]

Codecov Report

Merging #3033 (f3ec9cd) into main (4fe8537) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3033      +/-   ##
==========================================
- Coverage   96.54%   96.54%   -0.01%     
==========================================
  Files         468      468              
  Lines       29184    29197      +13     
  Branches     3589     3591       +2     
==========================================
+ Hits        28177    28189      +12     
  Misses        827      827              
- Partials      180      181       +1     

[codspeed-hq]

CodSpeed Performance Report

Merging #3033 will not alter performance

_{⚠️ No base runs were found}

_{Falling back to comparing stillmatic:main (f3ec9cd) with main (2ecaf8a)}

Summary

✅ 12 untouched benchmarks

[patrick91]

Looks good so far, I've left just a couple of minor comments 😊

[patrick91]

Thank you so much!

I've merged main here, I'll merge the PR once the tests are green 😊

[botberry]

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