Encode square brackets in path segments

[eyupcanakman]

HttpUrl.Builder.addPathSegment() and setPathSegment() did not percent-encode [ and ] in path segments. RFC 3986 section 3.3 reserves brackets for IPv6 literals in the authority component, so they should be percent-encoded when they appear in path segments.

Added a PATH_SEGMENT_ENCODE_SET_RAW constant that includes [] and applied it to raw-input methods (addPathSegment, addPathSegments, setPathSegment). addEncodedPathSegment(), setEncodedPathSegment(), and URL parsing preserve literal brackets as before.

Fixes #6790

[eyupcanakman]

@swankjesse on your 2023 ask about confirming browsers. Browsers per WHATWG leave [ and ] alone in path segments (neither is in the path percent-encode set), so new URL("https://x/a[0]").pathname stays /a[0] in Chrome and Firefox. The fix here matches RFC 3986 and Java URI (which already rejects [ in paths) but differs from what browsers do, which CONTRIBUTING prefers.

Should I close this? addEncodedPathSegment() and parsing already preserve literal brackets, so the only behavioral change is on the raw-input methods.

[JakeWharton]

The "encoded" variants of the functions are the "raw" ones. These functions are the ones which are used by 99.9% of callers. We should fix these. Retrofit wants this!