LogRecord Attributes - avoid heap allocations

[cijothomas]

We recently decided to pre-allocate up to 8 attributes, to avoid reallocation/copy costs, but attributes are still always on the heap.
Opening an issue to track improving this, so that, up to 8 attributes can be stack allocated, and then fallback to heap for more attributes. This, along with #1919, allow one to write logs without any allocation when using synchronous exporters like etw/user_events that require no batching.

We could add a dependency on SmallVec or make a home-grown solution. For an example implementation, .NET's TagList can be referred, which optimizes for up to 8 attributes.

We may need a feature flag to control this, as some scenarios (eg: Kernel mode) has less stack available.

[lalitb]

fixed in #1965 . The feature flag as discussed in #2055 would be added as required. Will create a separate issue for supporting kernel mode applications.