chore: add xz as a system dep and add real extendr version

[JosiahParry]

This PR

• closes xz missing from documented SystemRequirements #457
• closes Specify extendr-api version in Cargo.toml #348

The version of of extendr-api is fetched from https://crates.io/api/v1/crates/extendr-api using jsonlite::read_json() if this fails for any reason it will always fall back to "*". The version is set in the options in .onLoad() and is fetched from the options when using use_extendr()

This also sets the MSRV of extendr into the SystemRequirements as well as xz.

As a follow up we can create an rextendr.msrv option to store the MSRV instead of hard coding it.

[codecov]

Codecov Report

❌ Patch coverage is 45.45455% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.02%. Comparing base (9db60d0) to head (197c542).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
R/zzz.R	0.00%	18 Missing ⚠️

❌ Your patch status has failed because the patch coverage (45.45%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

Files with missing lines	Coverage Δ
R/setup.R	90.24% <100.00%> (+1.67%)	⬆️
R/use_extendr.R	98.67% <100.00%> (+0.01%)	⬆️
R/zzz.R	0.00% <0.00%> (ø)

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