add impersonate-chrome and impersonate-ff features (#482)

[selfisekai]

initial support for curl-impersonate. allows linking with system-provided libcurl-impersonate-chrome or libcurl-impersonate-ff instead of libcurl. doesn't allow to choose the impersonation target within the code (see #482 for further goals), but the CURL_IMPERSONATE env variable passed to the binary is respected by the library.