Add support for Perplexity-style citations

[SplittyDev]

What

This PR adds support for Perplexity's citations list.

See https://docs.perplexity.ai/api-reference/chat-completions for the docs.
Note: While Perplexity declares the type as any[], in practice these are strings.

Why

In order to extend support to other compatible providers, as discussed in #259 (comment).

Affected Areas

Minimal non-breaking changes to ChatResult and ChatStreamResult.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[SplittyDev]

I'll update the tests shortly.

[nezhyborets]

Great! Let's also add a section to doc about supporting different providers. No need to sweat about it, just introduce the section and write a short note about the change in this PR. I'm going to put more general info there later, like "to use a different provider you can set host, specify model" etc

[nezhyborets]

@SplittyDev would you be able to do a fix so the check passes?

[SplittyDev]

@SplittyDev would you be able to do a fix so the check passes?

Sure, but it might take a few days. I don't have much time at the moment, but I'll get back to this as soon as I can