Add weak consistency option to DescribeNamespaceRequest (#775)

<!-- Describe what has changed in this PR -->
**What changed?**
Added weak_consistency boolean field to `DescribeNamespaceRequest`. When
set to `true`, the server may serve the response from an
eventually-consistent source (e.g. the in-memory namespace registry)
instead of reading through to persistence.

<!-- Tell your future self why have you made these changes -->
**Why?**
`DescribeNamespace` currently always reads through to the persistence
store. The new field lets clients opt into eventually-consistent reads
where read-after-write semantics aren't required, while preserving the
existing strong-consistency contract for operator/admin callers (default
behavior unchanged).

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**
None. Additive field with default `false`, preserving current behavior
for clients that don't set it.

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right after this PR was merged. -->
**Server PR**
temporalio/temporal#10103

Author: simvlad