Implement a bulk Identity/Trait upload endpoint

[dabeeeenster]

Use the async processor to queue these up and process them async.

Suggest a limit of 100 identities per request.

Implement in SDKs:

• [ ] python
• [ ] node
• [ ] ruby
• [ ] rust
• [x] go
• [ ] java
• [ ] .NET
• [ ] elixir

Add docs

• [ ] docs

[dabeeeenster]

To test and deploy!

[matthewelwell]

This has now been deployed to our Edge API. An example cURL request is here:

curl --location --request POST 'https://edge.api.flagsmith.com/api/v1/bulk-identities' \
--header 'Content-Type: application/json' \
-- header 'X-Environment-Key: <your key>' \
--data-raw '{
    "data": [
        {"identifier": "identity_1", "traits": [{"trait_key": "foo", "trait_value": "bar"}]},
        {"identifier": "identity_2", "traits": [{"trait_key": "foo", "trait_value": "bar"}]}
    ]
}'

We now need to work on implementing this across our server side SDKs. I've added a checklist to the description of this issue.

[matthewelwell]

As per a request from a customer, we should add the option to merge existing traits to this by adding a separate PUT endpoint at the same URI (with the same request body)