feat: enable bridges using env var

[dvikan]

Allow enabling of bridges using env var.

Example (php-fpm):

env[RSSBRIDGE_system_enabled_bridges] = GettrBridge,ThePirateBayBridge

Example (docker):

ENV RSSBRIDGE_SYSTEM_ENABLED_BRIDGES = GettrBridge,ThePirateBayBridge

Yes they are case-insensitive.

Also introduce a new config option: enabled_bridges.

Keeping the whitelist.txt logic to preserve existing installations.

#2058
@gileri @Bockiii

[Bockiii]

Hi,

The env setting is definitive, not additive right?

Base Install: the 9 default bridges are active
set env variable to "youtubebridge": only yt is active, not the default ones (even though they are still in the config)
set whitelist.txt to "facebookbridge": still only youtubebridge is active because whitelist.txt loses in the race condition, right? (because whitelist is checked first, sets the config setting, then env is checked and overwrites the config setting)

[dvikan]

you are correctly describing the behavior in the pr. the various config options entirely replaces the previous value. it is not addition.

ordering of which config takes precedence

1. default config
2. config
3. whitelist.txt
4. env

the logic and code is getting kinda complex but dont think there's any bugs in the pr

[Bockiii]

Yeah, didnt mean it as a bug, more an idea of what the wanted scenario is.

Okay, thats fine for me. Maybe that precedence should be noted somehow in the docu, otherwise the pr lgtm!

Thanks!