[DESIGN] Tags introduction [IOT-1359]

designs/0002-tags.md

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+# Introduction of tags for device and thing
+
+## Review Period
+
+Best before October, 18 2021.
+
+## What is the problem?
+We need to design the commands to **handle tags** for things and devices. Tags could be attached and detached to/from any thing or device. Also, certain commands could take tags as input in order to perform operations on multiple things/devices.
+
+## Out-of-Scope
+
+## User Experience Walkthrough
+
+
+## Implementation
+
+### Project Changes
+
+Commands that could regard multiple things or devices could be changed in order to accept tags.
+
+These commands can be:
+
+**list commands**:
+
+`thing list --tag <key0>=<value0>,<key1>=<value1>` to list all the things having all the tags passed
+
+`device list --tag <key0>=<value0>,<key1>=<value1>` to list all the devices having all the tags passed
+
+alternative: `--filter` instead of `--tag`
+
+
+**delete commands**:
+
+`thing delete --tag <key0>=<value0>,<key1>=<value1>` to delete all the things having all the tags passed
+
+`device delete --tag <key0>=<value0>,<key1>=<value1>` to delete all the devices having all the tags passed
+
+
+**ota command**:
+
+`ota upload --device-tag <key0>=<value0>,<key1>=<value1> --file <sketch-file.ino.bin>` to perform an upload via ota to all the devices having all the tags passed
+
+
+To be decided, **which flag is mandatory and which is optional**:
+In delete and ota commands, the `--id` flag should become **optional** instead of mandatory. 
+Then, if neither `--id` nor `--tag` is passed, the command could return an error telling the user to specify at least one of the two flags.
+On the other hand, if both flags are passed, we could follow different approaches: return an error, use only one flag (only tags or only id), try to use both flags.
+
+Another thing to be decided is **how to handle errors**:
+- Try to execute the command on all the indicated resources, without stopping in case of errors. At the end, print all the errors details.
+- Stop as soon as an error is encountered.
+
+### Breaking Change
+
+The changes listed above should not break anything, the commands could be used as before.
+
+### Design
+
+New commands should be introduced in order to add and delete tags to devices and things. Here are some alternatives:
+
+Implementing a tag command:
+`arduino-cloud-cli tag create --thing <thingID> --tags <key0>=<value0>,<key1>=<value1>`
+
+Adding a tag subcommand in device and thing commands:
+`arduino-cloud-cli thing create-tags --thing <thingID> --tags <key0>=<value0>,<key1>=<value1>` and same for device
+
+
+### Documentation Changes
+
+Readme should be updated
+
+## Additional Notes
+
+https://arduino.atlassian.net/jira/software/projects/IOT/boards/277?selectedIssue=IOT-1359