Allow 'Discovered' flows to have title placeholders

[Jc2k]

This PR is a bit of a proof of concept and more about starting a conversation than anything else.

When I switch homekit_controller over to Config Entries (hopefully in 0.94) even though i know the name and model of the device there doesn't seem to be a mechanism to surface that in the 'Discovered' part of the Integrations panel. Instead I just see the localisation for component.homekit_controller.config.title.

This PR allows me to push a placeholders dict via a flows context which I can then fill into a new flow_title localisation. If there are no placeholders it continues to use the existing title localisation so existing integrations are not impacted.

This works, and doesn't require any changes on the server side for existing integrations. For integrations that want to do it, they can just poke a placeholders dict into self.context from their async_step_discovery and add a flow_title to their strings.json.

However it does feel like i'm breaking an abstraction or violating some layering by doing it this way so I thought it best to start a conversation about the "right way". Should I added a new key at the context level? Should there (is there?) a nicer way to set context during async_step_discovery? Should a flow have an optional title that can be concatenated together with the integration name (I think thats what happens with a config entry's title, for example).

@balloob what do you think? should i be doing this a different way?

[balloob]

The code starting a config flow should not be aware of what it is discovering, nor be aware what keys to set in the context. I think that indeed it should be dealt with by setting something during async_step_discovery. I think that allowing a config flow write to it's own context is not such a bad idea.

[balloob]

And it's actually a great idea for a lot of other discoveries too.

[balloob]

Can we call it title_placeholders

[Jc2k]

Requested change made.

On the HA server side i'm currently doing something like this inside async_step_discovery:

self.context['title_placeholders'] = {
    'name': discovery_info['name'],
}

And in my strings:

{
    "config": {
        "title": "HomeKit Accessory",
        "flow_title": "HomeKit Accessory: {name}",
<snip>

This works but it's throwing pylint off:

E1137: 'self.context' does not support item assignment (unsupported-assignment-operation)

I could suppress it but I don't really know why it thinks this is a problem.

[balloob]

I think it's because of the type of context. You can disable pylint check