wifi(API): New API surface for connection via NetworkRequest

Adds a new builder class build a NetworkSpecifier that can be
used along with a standard ConnectivityManager.requestNetwork() to
request connection to a specific wifi network.
To avoid referencing any params from WifiConfiguration class (which is
going to be deprecated), copy over all public non-deprecated constants
to the new builder class.
Note: Most of these fields will be needed for |NetworkSuggestion| as well,
hence moving it to this common builder.

Since all the internal modules of wifi stack use WifiConfiguration,
WifiNetworkSpecifier embeds a standard WifiConfiguration field to hold
the relevant network credentials. There are 2 fields added in
WifiNetworkSpecifier to allow apps to specify a SSID/BSSID pattern to
match.

Bug: 113878056
Test: Unit tests
Test: `make api-stubs-docs-update-current-api`
Change-Id: I9c8919deb23024123f2b69c6fd705092976a1acf