feat(body): add UnsyncBoxBody::new constructor and From<ResponseBody>

[jlizen]

Supersedes: #537

Motivation

Users who return responses with different body types (e.g., from multiple match branches) need to box them into a common type. When one branch uses ServeDir/ServeFile, the response body is already internally boxed, but wrapping it in UnsyncBoxBody::new() causes double-boxing (double allocation + double indirection).

PR #537 by @nanoqsh added From impls directly on http_body_util::combinators::UnsyncBoxBody, but tower-http has a policy of not leaking http_body_util types in its public API.

Solution

Expose tower-http's own UnsyncBoxBody wrapper with:

1. A public new() constructor that type-erases any Body impl (analogous to http_body_util::combinators::UnsyncBoxBody::new)
2. A From<ServeFileSystemResponseBody> impl that unwraps the already-boxed inner body without re-boxing

Usage becomes:

// Before (double-boxing):
.map(|res| res.map(UnsyncBoxBody::new))

// After (zero-cost conversion from ServeDir response):
.map(|res| res.map(UnsyncBoxBody::from))

The internal pub(crate) fn new() that previously accepted the raw http_body_util inner type was renamed to from_inner() to avoid ambiguity with the new public constructor.

Tests verify that UnsyncBoxBody::new correctly boxes a body and that the From<ResponseBody> conversion produces a valid, readable body from a real ServeDir response. The conversion is zero-cost by construction (it just moves the inner field), so correctness is primarily a compile-time guarantee.

[jlizen]

@seanmonstar / @davidpdrsn - either of y'all have time to review this?