Bluetooth: host: Add application control of data length procedure

Add application control of initiating data length procedure and it's
parameters.
The reasons for allowing the application control over the data length:
 - Bandwidth control adjusted based on number of active connections.
 - Changing data length before switching to coded phy.
 - Applying workarounds for interoperability problems.
 - Controlling order of ATT MTU and data length procedures.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>