[bwplotka]

Adds the standalone http-config Cargo feature and expfmt::http_config module implementing Prometheus HTTPClientConfig schema parsing, validation, set_directory, secret redaction in Debug and Serialize, and reqwest (rustls + http2) client and request builders reusable across HTTP service discovery and scraping. Part of prometheus/scrape.rs#36.

Stack:

1. discovery: add TargetGroup, StaticConfig, and Prometheus duration helpers #39 discovery: add TargetGroup, StaticConfig, and Prometheus duration helpers
2. discovery/dns: add DNS service discovery (dns_sd_configs) #43 discovery/dns: add DNS service discovery (dns_sd_configs)
3. http_config: add Prometheus HTTPClientConfig and reqwest client builder #44 http_config: add Prometheus HTTPClientConfig and reqwest client builder (this PR)
4. discovery/kubernetes: add pod service discovery (role: pod) #45 discovery/kubernetes: add pod service discovery (role: pod)

Commits in reading order:

1. 64ace43: http_config: add Prometheus HTTPClientConfig schema, validation, and secret redaction
2. ed9f909: http_config: add reqwest client builder, header injection, and HTTPS mTLS tests

Intentional differences from Go

• HttpClientConfig builds a reqwest::Client backed by rustls rather than wrapping Go net/http.RoundTripper middleware as in config/http_config.go#L604.
• oauth2 in config/http_config.go#L239, external secret manager *_ref fields in config/http_config.go#L505, tls_config.server_name in config/http_config.go#L1305-L1306, and combining proxy_from_environment with proxy_connect_header in config/http_config.go#L1687-L1692 are parsed and validated for config compatibility, and return HttpConfigError::InvalidConfig at client build or request time until runtime support is wired.