feat: allow passing HTTP headers to SvgUri

[patrickwehbe]

Summary

Adds an optional headers prop to the URI-based components so requests for remote SVGs can carry custom HTTP headers (for example an Authorization token, or a header that tells the server which variant to return). This is the feature requested in #2320.

The prop is modelled on React Native's ImageURISource.headers, so it should feel familiar to anyone who already sets headers on <Image source={{ uri, headers }} />.

Changes

• fetchText / fetchUriData (src/utils/fetchData.ts) take an optional headers map and forward it to fetch(uri, { headers }).
• headers?: Record<string, string> is added to UriProps in src/xml.tsx.
• The prop is passed through from SvgUri, SvgFromUri, SvgCssUri and SvgWithCssUri. The effect dependency array and the class componentDidUpdate checks include headers, so swapping the headers re-fetches.
• Added apps/common/test/Test2320.tsx demonstrating the prop, and registered it in the test index.

Notes

• This is opt-in and fully backward compatible. When headers is omitted, fetch is called exactly as before.
• As noted in the issue, callers should keep the headers object referentially stable (e.g. useMemo) to avoid re-fetching on every render. The example does this.
• Local SVGs loaded via require() don't go through the network, so they're unaffected.

Test plan

• yarn tsc passes.
• yarn lint passes.
• The new example type-checks against apps/common.

Closes #2320