bluez5: media-source: add option to control the target latency of the decode-buffer

On production systems, having a constant high latency is favored over
dynamically adjusting it in order to optimize for low latency,
because every time a dynamic adjustment happens, there's a glitch.

This adds an option to let the user specify the exact amount of latency
they want.

doc/dox/config/pipewire-props.7.md

@@ -1196,6 +1196,11 @@ this instance. Available values:
   - input: appear as source node.
 \endparblock
 
+@PAR@ node-prop  bluez5.decode-buffer.latency   # integer
+Applies on media source nodes and defines the target amount
+of samples to be buffered on the output of the decoder.
+Default: 0, which means it is automatically determined.
+
 @PAR@ node-prop  node.latency-offset-msec   # string
 Applies only for BLE MIDI nodes.
 Latency adjustment to apply on the node. Larger values add a