support `Strict-Transport-Security`, `Content-Security-Policy` and `X-Frame-Options`
HTTP headers in all VictoriaMetrics components.
The values for headers can be specified by users via the following flags:
`-http.header.hsts`, `-http.header.csp` and `-http.header.frameOptions`.
Co-authored-by: hagen1778 <roman@victoriametrics.com>
* lib/httpserver: added tlsCipherSuites flag
* lib/httpserver: compare lower case strings
* lib/httpserver: use EqualFold
* lib/httpserver: used flagutil.NewArray, supported only strings cipher suites
* lib/httpserver: updated flag description, added flag to documentation
* Update lib/httpserver/httpserver.go
Co-authored-by: Aliaksandr Valialkin <valyala@victoriametrics.com>