move `RequestUserAgent` to `internal/useragent`

[cuixq]

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 64.70588% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 64.03%. Comparing base (055ef05) to head (64ea62f).

Files	Patch %	Lines
internal/resolution/client/npm_registry_client.go	0.00%	2 Missing :warning:
pkg/osv/osv.go	66.66%	2 Missing :warning:
pkg/osvscanner/osvscanner.go	0.00%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #981      +/-   ##
==========================================
- Coverage   64.63%   64.03%   -0.60%     
==========================================
  Files         146      146              
  Lines        9308    11977    +2669     
==========================================
+ Hits         6016     7670    +1654     
- Misses       2838     3854    +1016     
+ Partials      454      453       -1     

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