Make timeouts in the HTTP client configurable

The [`NewTLSClient` function](https://github.com/grafana/alerting/blob/3c7640592aa884caf050ca19e515838d402fd6b1/http/tls.go#L10-L31) returns an `*http.Client` with hard-coded timeouts.

```go
return &http.Client{
Timeout: time.Second * 30,
Transport: &http.Transport{
TLSClientConfig: tlsConfig,
Proxy: http.ProxyFromEnvironment,
Dial: (&net.Dialer{
Timeout: 30 * time.Second,
}).Dial,
TLSHandshakeTimeout: 5 * time.Second,
},
}
```

We can make these timeouts configurable via arguments, allowing users to tweak these values to their needs.

Start in http/tls.go at NewTLSClient and review the hard-coded HTTP client, dialer, and TLS handshake timeouts shown in the issue. Trace how this function is called, expose the timeout values through its arguments, and verify that callers can configure them without changing the intended defaults.

Written by the indexing model from the issue text.