Support sending gcode or SL1 to PrusaLink machines

[Floppy]

Resolves #6435.

This PR is a simple example of how to implement a new print API service. If you want to implement another one, follow these steps:

1. Copy the spec file and change the various names at the top of the file - most of the actual tests shouldn't need changing, at least at first.
2. Put endpoint and credentials in .env.test.local so you can talk to the API
3. If your API is running on localhost, temporarily comment out config.ignore_localhost = true in spec/support/vcr.rb but don't commit it
4. Copy a print service in app/services/print and implement the changes for your new service
5. API interactions will be recorded by vcr so it can be tested without the real API. Remove the :vcr metadata tag at the start of your test file to disable this while you're developing, but put it back when you're done to record the test interactions. Make sure the filtering has worked and that the credentials you've used for development aren't in the cassettes.
6. Run bundle exec i18n-tasks add-missing --nil-value. That should add the new translation key for your service name. Set the name in en.yml and leave the rest blank. I actually forgot to do this in this PR because I forgot to change the i18n-tasks-use line; see Improve and fix translation strings for printing feature #6452 for the changes to expect.
7. Once it's all working, remove the credentials set in .env.test.local and make sure the tests pass against the cassettes.
8. Run bundle exec rubocop -a to autofix any style differences.
9. Open a PR, and have a cup of tea to celebrate 🎉

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 14 complexity

Metric	Results
Complexity	14

View in Codacy

🟢 Coverage 100.00% diff coverage · +0.07% coverage variation

Metric	Results
Coverage variation	✅ +0.07% coverage variation (-1.00%)
Diff coverage	✅ 100.00% diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (266313c)	7385	6170	83.55%
Head commit (68306d1)	7416 (+31)	6201 (+31)	83.62% (+0.07%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#6432)	31	31	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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