Fetch `maven-metadata.xml` from Maven Central

[cuixq]

https://github.com/google/osv-scanner/issues/1127 and https://github.com/google/osv-scanner/issues/1045

We need maven-metadata.xml for the list of available versions as well as snapshot versions.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 58.00000% with 21 lines in your changes missing coverage. Please review.

Project coverage is 65.87%. Comparing base (ecd7cc8) to head (83fcad5).

Files	Patch %	Lines
internal/resolution/datasource/maven_registry.go	58.00%	15 Missing and 6 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1160      +/-   ##
==========================================
+ Coverage   65.86%   65.87%   +0.01%     
==========================================
  Files         168      168              
  Lines       14076    14116      +40     
==========================================
+ Hits         9271     9299      +28     
- Misses       4293     4301       +8     
- Partials      512      516       +4     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[ifuatgucluer]

good job