Quadratic run time when checking a start tag for duplicate attribute names

`BytesStart::attributes()` returns an `Attributes` iterator which, by default
(`with_checks(true)`), rejects a start tag that repeats an attribute name. For
each attribute yielded, the iterator compared the new name against every name
seen so far in the same tag using a linear scan, so a start tag with `N`
distinct attribute names cost `O(N²)` byte comparisons. There was no bound on
`N` other than the size of the buffered start tag.

## Impact

Any code that parses untrusted XML and iterates a start tag's attributes with
the default duplicate check enabled can be made to spend CPU time quadratic in
the number of attributes on a single tag. Because the check is pure computation
with no `.await`/I/O, an I/O-based timeout on the consumer (for example a read
or request timeout) cannot interrupt it while it runs.

Measured cost of a single start tag, release build:

| Attributes on one tag | Time |
|---|---|
| 80,000 | ~6 s |
| 800,000 | ~10 min |

The cost grows with the square of the attribute count, so a start tag of a few
tens of megabytes can stall a parsing thread for hours. No memory is exhausted
and the parser does not crash; the effect is CPU exhaustion on the thread doing
the parsing: a single crafted start tag can pin a CPU core for minutes to hours,
denying service to that worker. A deployment that places a wall-clock bound on
parsing, or confines it to a non-critical thread, may consider the availability
impact lower.

## Affected code paths

* `BytesStart::attributes()` / `Attributes` iterated with checks enabled (the
default), and `BytesStart::try_get_attribute`.
* `NsReader`, which resolves namespaces by iterating a tag's attributes and so
reaches the same check internally.

Consumers that iterate attributes with `.attributes().with_checks(false)` and do
not use `NsReader` are not affected.

This was reported as reachable by a remote, unauthenticated attacker in a
real-world RPKI relying party (NLnet Labs Routinator) via a crafted RRDP
`snapshot.xml`.

## Remediation

Upgrade to `quick-xml >= 0.41.0`, where the duplicate check keeps the linear
scan for start tags with a small number of attributes and switches to an `O(1)`
hash pre-filter above a threshold, making the whole tag `O(N)`. The reported
`AttrError::Duplicated` positions are unchanged.

If upgrading is not possible and duplicate-name detection is not required,
disable it with `.attributes().with_checks(false)` (this does not help
`NsReader` consumers, which have no equivalent opt-out before 0.41.0).