perf: Avoid shallow copies of DSC when `sample_rate` or `replay_id` are overwritten

[Flash0ver]

Description

So far, the Dynamic Sampling Context (DSC) was immutable after the first propagation.

But in the meantime, immutability only partially applies, after

• feat: Associate replays with errors and traces on Android #4133
• fix: overwrite DSC sample_rate when sampling #4374

For both cases, we allocate a shallow copy of the DynamicSamplingContext instance, and the underlying IReadOnlyDictionary.
Since DynamicSamplingContext is an internal class, we can optimize the implementation details to avoid copy allocations, and instead mutate the internal Dictionary that we create in the .ctor.

Alternatively, we could rewrite or derive a DynamicSamplingContext, and/or introduce something similar like a DynamicSamplingContextBuilder. But since the DynamicSamplingContext is internal this may not be required.

[linear]

NET-389 perf: Avoid shallow copies of DSC when `sample_rate` or `replay_id` are overwritten