[vazarkevych]

Summary

Adds updateAttributes / updateAttributesAsync, which merge into the existing attributes instead of replacing them. Split out of #176 (section 1).

Depends on #187 — this branch is built on it; only the last commit belongs to this PR.

API	Behavior
setAttributes / setAttributesAsync	replace: anything not included is dropped (unchanged)
updateAttributes / updateAttributesAsync	merge: new keys added, existing overwritten, untouched preserved

Why: adding one attribute (e.g. after login) meant rebuilding the whole map, and a key left out was silently dropped.

Behavior

Same as updateAttributes in the JS SDK (GrowthBook.ts): a shallow merge, then the regular setAttributes path. So sticky bucket refresh and, in remote-eval mode, the fresh evaluation from #187 apply unchanged; the async variant awaits both.

• The merge is shallow: a nested Map or List replaces the previous value of that key.
• A null value is stored as null; it does not remove the key.
• The caller's Map is not modified.

setAttributes now documents that it is a full replace and points to updateAttributes.

Compatibility

Additive only: no existing API or behavior changes.

Test plan

• 13 new tests. 11 in gb_sdk_attributes_test.dart: merge, overwrite, merge vs replace, empty map, null values, shallow merge, bucketing on merged attributes, caller's Map untouched, accumulation, async variant, sticky bucket refresh awaited. 2 in gb_sdk_remote_eval_test.dart: the remote-eval POST carries the merged attributes, for both variants.
• 10 of the new tests fail if the merge is replaced by a plain replace; the other 3 cover properties a replace also has.
• Against Flutter 3.27.0 (the CI version): dart analyze --fatal-infos, dart format and flutter test (3 runs) clean.
• Manual check against a GrowthBook Proxy with remote evaluation and streaming: updateAttributes({'country': 'UA'}) sent a POST with {id, country}, kept the id-based experiment assignment, and a later published change still arrived through the stream with the merged attributes.