[Diagnostics] RAM usage diagnostic sources

[avtolstoy]

submission notes

**Important:** Please sanitize/remove any confidential info like usernames, passwords, org names, product names/ids, access tokens, client ids/secrets, or anything else you don't wish to share.

Please Read and Sign the Contributor License Agreement ([Info here](https://github.com/spark/firmware/blob/develop/CONTRIBUTING.md)).

You may also delete this submission notes header if you'd like. Thank you for contributing!

NOTE: This PR targets feature/diagnostics-merge branch

Solution

This PR adds the following diagnostic sources:

• Total RAM: ID 25, Type: scalar(32)
  • Total amount of RAM available on the device in bytes. This value normally equals the size of the region shared by the heap and user application static RAM. Does not include static RAM in use by the system parts.
• Used RAM: ID 26, Type: scalar(32)
  • Amount of RAM used out of "Total RAM" (user application static RAM + heap used).

Steps to Test

• d command in listening mode

Example App

N/A

References

• [CH8737]

---

Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[pkourany]

@avtolstoy, what is missing is a "largest allocatable chuck" tool which could be an indicator for the amount of fragmentation in the heap.