Berry add 'gc_heap' and 'gc_time' to 'tasmota.memory()'

[s-hadinger]

Description:

Berry, add the following fields to tasmota.memory():

• gc_time: time in milliseconds of the last garbage collection
• gc_heap: free heap memory when the last garbage collection happened

tasmota.memory("gc_heap") provides a more consistent view of the memory because you don't get the jitter of objects allocated but not yet freed.

Checklist:

• The pull request is done against the latest development branch
• Only relevant files were touched
• Only one feature/fix was added per PR and the code change compiles without warnings
• The code change is tested and works with Tasmota core ESP8266 V.2.7.8
• The code change is tested and works with Tasmota core ESP32 V.3.1.4
• I accept the CLA.

NOTE: The code change must pass CI tests. Your PR cannot be merged unless tests pass