allow library users to specify a user agent

[spencerschrock]

osv-scanner v1 allowed library users to specify a user agent via the RequestUserAgent variable.
https://pkg.go.dev/github.com/google/osv-scanner@v1.9.2/pkg/osv#RequestUserAgent

RequestUserAgent was deleted in #1470 and the user agent is now hardcoded as osv-scanner_scan/<version>

osv-scanner/pkg/osvscanner/osvscanner.go

Lines 145 to 148 in 0dd3348

	// Online Mode
	// -----------
	// --- Vulnerability Matcher ---
	externalAccessors.VulnMatcher = osvmatcher.New(5*time.Minute, "osv-scanner_scan/"+version.OSVVersion, actions.HTTPClient)