fix(firestore): add noexcept specifiers to move, swap, hash operations - #16117

b/269108721

Adds `noexcept` specifiers to move, swap, and hash operations throughout
the SDK.

According the C++ Core Guidelines, move, swap, and hash operations
should be `noexcept`.
If move operations are throwable, many std lib containers that use these
objects will fall back to using copy semantics, degrading performance.

I used grep to attempt to locate all instances of these operations
that did not have the `noexcept` specifiers.

Command:
```
rg --pcre2 -n -g '*.{h,cc}' '\b(?:friend\s+)?(?:void|size_t|std::size_t|auto)\s+(?:swap|hash|operator\s*\(\))\s*\(' | rg -v 'noexcept'
```

Results:

Deleted constructors. No action required.
- util/statusor_internals.h:248:  TraitsBase(TraitsBase&&) = delete;
- util/statusor_internals.h:250:  TraitsBase& operator=(TraitsBase&&) = delete;
- util/ordered_code.h:155:  OrderedCode(OrderedCode&&) = delete;
- util/ordered_code.h:156:  OrderedCode& operator=(OrderedCode&&) = delete;
- remote/datastore.h:154:  Datastore(Datastore&& other) = delete;
- remote/datastore.h:156:  Datastore& operator=(Datastore&& other) = delete;
- util/testing_hooks.h:155:  TestingHooks(TestingHooks&&) = delete;
- util/testing_hooks.h:157:  TestingHooks& operator=(TestingHooks&&) = delete;
- local/memory_document_overlay_cache.h:39:  MemoryDocumentOverlayCache(MemoryDocumentOverlayCache&&) = delete;
- local/memory_document_overlay_cache.h:40:  MemoryDocumentOverlayCache& operator=(MemoryDocumentOverlayCache&&) = delete;
- local/leveldb_document_overlay_cache.h:51:  LevelDbDocumentOverlayCache(LevelDbDocumentOverlayCache&&) = delete;
- local/leveldb_document_overlay_cache.h:52:  LevelDbDocumentOverlayCache& operator=(LevelDbDocumentOverlayCache&&) = delete;

Default constructors. Are implicitly noexcept. Explicitly make noexcept.
[x] remote/bloom_filter.h:35:  BloomFilter(BloomFilter&&) = default;
[x] remote/bloom_filter.h:37:  BloomFilter& operator=(BloomFilter&&) = default;
[x] util/statusor_internals.h:230:  TraitsBase(TraitsBase&&) = default;
[x] util/statusor_internals.h:232:  TraitsBase& operator=(TraitsBase&&) = default;
[x] util/statusor_internals.h:239:  TraitsBase(TraitsBase&&) = default;
[x] util/statusor_internals.h:241:  TraitsBase& operator=(TraitsBase&&) = default;
[x] util/random_access_queue.h:59:  RandomAccessQueue(RandomAccessQueue&& other) = default;
[x] util/random_access_queue.h:68:  RandomAccessQueue& operator=(RandomAccessQueue&& other) = default;
[x] api/settings.h:52:  Settings(Settings&& other) = default;
[x] api/settings.h:55:  Settings& operator=(Settings&& other) = default;

Utils copied, this is not code written by us. No action required.
[ ] util/status.h:140:      void operator()(const State* ptr) const;
[ ] util/statusor.h:166:  StatusOr& operator=(Status&& status);
[ ] util/statusor.h:243:StatusOr<T>& StatusOr<T>::operator=(Status&& status) {
[ ] util/statusor.h:116:  StatusOr(StatusOr&&) = default;
[ ] util/statusor.h:117:  StatusOr& operator=(StatusOr&&) = default;

[x] model/document_key.h:113:  size_t operator()(const DocumentKey& key) const;
[x] model/overlay.h:79:  std::size_t operator()(const Overlay&) const;
   - could be an issue if Mutation is not nothrow hashable. It hashes type, key, mutation, precondition. I think these should all be nothrow hashable.
[ ] model/field_path.cc:84:  void operator()(T* out, const std::string& segment) {
   - i don't think this can be made noexcept, since it's templated. T-> could throw an exception.
[x] core/query.h:332:  size_t operator()(const firebase::firestore::core::Query& query) const {
   - Query::Has() -> absl::StrCat() -> std::badalloc in OOM. Already fatal. std::terminate ok. use noexcept.
[x] core/pipeline_util.h:204:  size_t operator()(
   - Query::Has() -> absl::StrCat() -> std::badalloc in OOM. Already fatal. std::terminate ok. use noexcept.
[x] core/pipeline_util.h:212:  size_t operator()(
   - Query::Has() -> absl::StrCat() -> std::badalloc in OOM. Already fatal. std::terminate ok. use noexcept.
[x] core/target.h:245:  size_t operator()(const firebase::firestore::core::Target& target) const {