Chunk 10 of CMP contribution to OpenSSL: CMP http client and related tests

Chunk 10 of CMP contribution to OpenSSL: CMP http client and related tests

    Also improve the generic HTTP client w.r.t. proxy and no_proxy options.

    Certificate Management Protocol (CMP, RFC 4210) extension to OpenSSL
    Also includes CRMF (RFC 4211) and HTTP transfer (RFC 6712).
    Adds the CMP and CRMF API to libcrypto and the "cmp" app to the CLI.
    Adds extensive documentation and tests.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
(Merged from https://github.com/openssl/openssl/pull/11404)