Mavlink: implemented SCALED_PRESSURE streams

[dayjaby]

Describe problem solved by this pull request
mavros expects SCALED_PRESSURE at the moment to send the barometer temperature. With this PR, PX4 is able to send SCALED_PRESSURE/SCALED_PRESSURE2/SCALED_PRESSURE3.

Describe your solution
We subscribe directly to sensor_baro and forward the absolute pressure and temperature.

Describe possible alternatives
The template can be removed if SCALED_PRESSURE would contain a sensor index, similar to mavlink/rfcs#13.

Test data / coverage

make px4_sitl_default gazebo_standard_vtol
mavlink stream -u 18570 -s SCALED_PRESSURE -r 50

[dagar]

The change looks good, it's just pushing px4_fmu-v2 over the flash limit. Let me see what we can do.

[dayjaby]

The change looks good, it's just pushing px4_fmu-v2 over the flash limit. Let me see what we can do.

With the new commit, I could reduce the size a bit by adding a constexpr qualifier to get_name_static and get_id_static. I also commented out SCALED_PRESSURE2 and SCALED_PRESSURE3, as my goal at the moment is to get the barometric temperature via mavros which needs only SCALED_PRESSURE. They can easily be added later.

Before this PR:

Memory region         Used Size  Region Size  %age Used
           flash:     1656085 B      2032 KB     79.59%
            sram:       31796 B       192 KB     16.17%
          ccsram:          0 GB        64 KB      0.00%

Without the latest commit:

Memory region         Used Size  Region Size  %age Used
           flash:     1656277 B      2032 KB     79.60%
            sram:       31796 B       192 KB     16.17%
          ccsram:          0 GB        64 KB      0.00%

With the latest commit:

Memory region         Used Size  Region Size  %age Used
           flash:     1655621 B      2032 KB     79.57%
            sram:       31796 B       192 KB     16.17%
          ccsram:          0 GB        64 KB      0.00%

[dagar]

Nice one! Looks like it now more than pays for itself on fmu-v2.