Sync MQTT files from C-SDK for commit e60683b

[aggarw13]

Copy all files under aws-iot-device-sdk-embedded-C/tree/development/libraries/standard/mqtt folder for commit ID e60683b

Note: Subsequent PRs will bring in third-party tool dependencies, re-add LICENSE/README/CONTRIBUTING files, re-structure folder, rename files and remove integration tests, and provide functioning CMake, doc and CBMC targets.

Diff with commit on C-SDK repository:
The only difference between mqtt folder in C-SDK and this PR is the presence of the CBMC REAMDE files. These files are symlinked in C-SDK, and these symlinks will be enabled in a subsequent PR that brings CBMC third-party dependency

╭─ubuntu@ip-172-31-24-209 ~/Repos/coreMQTT  ‹sync/c-sdk-commit-d36047b› 
╰─➤  diff -r ../aws-iot-device-sdk-embedded-C/libraries/standard/mqtt/ .
Only in .: .git
diff: ./cbmc/include/README.md: No such file or directory
diff: ./cbmc/proofs/Makefile.common: No such file or directory
diff: ./cbmc/proofs/README.md: No such file or directory
diff: ./cbmc/proofs/prepare.py: No such file or directory
diff: ./cbmc/sources/README.md: No such file or directory
diff: ./cbmc/stubs/README.md: No such file or directory

[sarenameas]

Do we want to delete these files?

[aggarw13]

I will re-add these files in a separate PR with the correct content.
The purpose of this PR is to only have the master branch contain the same code as that under libraries/standard/mqtt from C-SDK