bluetooth: host: Add support for LE Connection Subrating

Adds support for LE Connection Subrating as defined in Core 5.4
Vol 6, Part B, Section 5.1.19.
As this is primarily a controller feature, the host support is mostly
a wrapper around the relevant HCI commands.

Note that subrating provides a new method to update the connection's
peripheral latency and supervision timeout alongside subrating parameters.

Signed-off-by: Aleksandar Stanoev <aleksandar.stanoev@nordicsemi.no>