S3 backend: add several http client override options

[wojas]

Add several options to override default http client parameters.

We used to use the defaults provided by tlsConfig.HTTPClient(), but recently hit situations where we want to override those. For this we need to create the objects ourselves, because the options are hidden behind interfaces.

	// IdleConnTimeout is the maximum amount of time an idle
	// (keep-alive) connection will remain idle before closing
	// itself. Default if unset: 90s
	IdleConnTimeout time.Duration `yaml:"idle_conn_timeout"`

	// MaxIdleConns controls the maximum number of idle (keep-alive)
	// connections. Default if unset: 100
	MaxIdleConns int `yaml:"max_idle_conns"`

	// DialTimeout is the maximum amount of time a dial will wait for
	// a connect to complete. Default if unset: 10s
	DialTimeout time.Duration `yaml:"dial_timeout"`

	// DialKeepAlive specifies the interval between keep-alive
	// probes for an active network connection. Default if unset: 10s
	DialKeepAlive time.Duration `yaml:"dial_keep_alive"`

	// TLSHandshakeTimeout specifies the maximum amount of time to
	// wait for a TLS handshake. Default if unset: 10s
	TLSHandshakeTimeout time.Duration `yaml:"tls_handshake_timeout"`

	// ClientTimeout specifies a time limit for requests made by this
	// HTTP Client. The timeout includes connection time, any
	// redirects, and reading the response body. The timer remains
	// running after Get, Head, Post, or Do return and will
	// interrupt reading of the Response.Body.
	// Default if unset: 15m
	ClientTimeout time.Duration `yaml:"client_timeout"`

[ahouene]

Looks good, thank you!