Bluetooth: ATT: Internalize ATT PDU TX pool

Why?
- metadata is easier to manage as an array + index
  - less error-prone -> less memory-management bugs
- we can. because of the previous refactor
- PDU allocations are more predictable
- ATT buffer size can be optimized by app
- isolates ATT from the rest of the ACL users
  - decouples ATT PDU size from e.g. SMP w/ LESC

Drawbacks:
- higher memory usage
- kconfig change

The higher memory use is only temporary, as this will be followed-up
with more refactors that should bring it back down.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>
Co-authored-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>