refactoring kindling usage and improve dnstt config

[WendelHime]

This PR creates a package for handling kindling http client and places that refer to the http client now get notified to retrieve a new http client with new config available.
It also improve the list of DNSTT configurations available.

Out of scope:
This PR isn't loading the configurations since dnstt integration tests are still failing and getlantern/engineering#2872 should cover that

Resolve getlantern/engineering#2869

[Copilot]

Pull request overview

This PR refactors the handling of the Kindling HTTP client by introducing a dedicated kindling package that centralizes client management and enables dynamic updates. The key improvements include consolidating Kindling initialization logic, supporting runtime updates when DNSTT configurations change, and improving the structure of DNSTT configuration handling to support multiple configurations instead of a single one.

Key changes:

• Created a new kindling package that manages HTTP client lifecycle and notifies consumers when new configurations are available
• Modified DNSTT configuration parsing to support multiple configurations (dnsttConfigs array) instead of a single dnstt object
• Added mutex-protected HTTP client updates across API, config fetcher, and issue reporter components

Reviewed changes

Copilot reviewed 10 out of 15 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
radiance.go	Simplified Radiance initialization by delegating Kindling setup to the new package
kindling/client.go	New package file implementing centralized HTTP client management with event-based updates
kindling/dnstt/parser.go	Refactored to parse multiple DNSTT configurations and emit update events
kindling/dnstt/parser_test.go	Updated tests to reflect new multi-config structure
api/api.go	Added mutex and event subscription to handle HTTP client updates
api/user.go	Added mutex locks around HTTP client usage in user-related API calls
api/subscription.go	Added mutex locks around HTTP client usage in subscription-related API calls
config/fetcher.go	Added mutex and event subscription for HTTP client updates
issue/issue.go	Added mutex and event subscription for HTTP client updates
config/fetcher_test.go	Updated import path for fronted package

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[garmr-ulfr]

A couple of comments, otherwise, LGTM.

[myleshorton]

Definitely opposed to adding all this locking and storing of http client state. I would revert the change here:

https://github.com/getlantern/kindling/pull/22/changes#diff-6b0f41a9a52c4516297a0a7d67b2a60f85783f9cc33b6234dc32e5759f1118deR96

that also breaks all other uses of DefaultClient, and just create call NewHTTPClient dynamically whenever we need one.

tbh I also don't think we should go to such lengths to support DNSTT config changes from the config server. why DNSTT and not other transports?