esp32,esp8266: Add support to set/get power saving mode of WLAN.

For esp32 and esp8266 this commit adds:
- a 'pm' option to WLAN.config() to set/get the wifi power saving mode; and
- PM_NONE, PM_PERFORMANCE and PM_POWERSAVE constants to the WLAN class.

This API should be general enough to use with all WLAN drivers.

Documentation is also added.

docs/library/network.WLAN.rst

@@ -133,4 +133,20 @@ Methods
    hostname       The hostname that will be sent to DHCP (STA interfaces) and mDNS (if supported, both STA and AP). (Deprecated, use :func:`network.hostname` instead)
    reconnects     Number of reconnect attempts to make (integer, 0=none, -1=unlimited)
    txpower        Maximum transmit power in dBm (integer or float)
+   pm             WiFi Power Management setting (see below for allowed values)
    =============  ===========
+
+Constants
+---------
+
+.. data:: WLAN.PM_PERFORMANCE
+        WLAN.PM_POWERSAVE
+        WLAN.PM_NONE
+
+    Allowed values for the ``WLAN.config(pm=...)`` network interface parameter:
+
+        * ``PM_PERFORMANCE``: enable WiFi power management to balance power
+          savings and WiFi performance
+        * ``PM_POWERSAVE``: enable WiFi power management with additional power
+          savings and reduced WiFi performance
+        * ``PM_NONE``: disable wifi power management