feat(csharp): implement FeatureFlagCache (WI-3.1) (#165)

## 🥞 Stacked PR
Use this
[link](https://github.com/adbc-drivers/databricks/pull/165/files) to
review incremental changes.
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[**stack/wi-3.1-feature-flag-cache**](#165)
[[Files
changed](https://github.com/adbc-drivers/databricks/pull/165/files)]
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[stack/wi-3.4-databricks-telemetry-exporter](#166)
[[Files
changed](https://github.com/adbc-drivers/databricks/pull/166/files/2262e9d512776719d02355190bd8824522537c6a..109f1af5faf78b5fbbdf4620a321b811266f63af)]
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[stack/wi-3.5-metrics-aggregator](#167)
[[Files
changed](https://github.com/adbc-drivers/databricks/pull/167/files/109f1af5faf78b5fbbdf4620a321b811266f63af..74af04194edcbce64f66efdf45944db1ed384727)]
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[stack/wi-3.5-circuit-breaker-manager](#168)
[[Files
changed](https://github.com/adbc-drivers/databricks/pull/168/files/74af04194edcbce64f66efdf45944db1ed384727..d0385b2145cda82c5fe0fc8311b0d64e9a6b05bc)]
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[stack/e2e-feature-flag-cache-tests](#169)
[[Files
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[stack/databricks-activity-listener](#170)
[[Files
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[stack/circuit-breaker-telemetry-exporter](#171)
[[Files
changed](https://github.com/adbc-drivers/databricks/pull/171/files/200014e9b7557de490944e5889fb197dc00edb41..e886588830c528ab644b38004337e7bfc1f12c31)]
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[stack/telemetry-client-manager-wi-3.2](#172)
[[Files
changed](https://github.com/adbc-drivers/databricks/pull/172/files/e886588830c528ab644b38004337e7bfc1f12c31..7d1e0741f4942c15e8b68f64a8199ebdae678758)]
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[stack/telemetry-client-wi-5.5](#173)
[[Files
changed](https://github.com/adbc-drivers/databricks/pull/173/files/7d1e0741f4942c15e8b68f64a8199ebdae678758..d2a7725cd098a05ac0b9b6a0135c34823b18c1ab)]
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[stack/telemetry-client-manager-e2e-wi-7](#174)
[[Files
changed](https://github.com/adbc-drivers/databricks/pull/174/files/d2a7725cd098a05ac0b9b6a0135c34823b18c1ab..66a44b1a372e4c27c924002a47ef755a10e7eaec)]
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[stack/telemetry-client-e2e-tests-wi-7-standalone](#175)
[[Files
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[stack/wi-6.1-databricks-connection-telemetry-integration](#176)
[[Files
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[stack/wi-6.2-telemetry-tags-driver-activities](#177)
[[Files
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[stack/wi-9-full-integration-e2e-tests](#178)
[[Files
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Co-authored-by: Jade Wang <jade.wang+data@databricks.com>
Co-authored-by: Claude <noreply@anthropic.com>

Author: 3 people

csharp/Directory.Packages.props

@@ -35,6 +35,8 @@
     <PackageVersion Include="System.Net.Http" Version="4.3.4" />
     <!-- Resilience -->
     <PackageVersion Include="Polly" Version="8.5.0" />
+    <!-- Caching -->
+    <PackageVersion Include="Microsoft.Extensions.Caching.Memory" Version="8.0.1" />
     <!-- Source Link -->
     <PackageVersion Include="Microsoft.SourceLink.GitHub" Version="8.0.0" />
   </ItemGroup>