Use of non-canonical MIME headers causes unnecessary string allocations in storage SDKs

[taywrobel]

Bug Report

• What happened?

The GMT time normalization for headers introduced in PR #20495 uses non-canonical MIME headers for the headers in the storage/azblob SDK:

• x-ms-source-if-modified-since (canonically X-Ms-Source-If-Modified-Since)
• x-ms-source-if-unmodified-since (canonically X-Ms-Source-If-Unmodified-Since)
• x-ms-immutability-policy-until-date (canonically X-Ms-Immutability-Policy-Until-Date)

net/http.Header.Get canonicalizes the header if it's not already in the canonical format. This results in 3 unnecessary allocations per query, which under systems with high concurrent load can add up.

Since this PR was merged the same autorest rules were applied to the storage/azdatalake SDK as well in PR #21387.

• What did you expect or want to happen?
  Use header keys which are already canonical to avoid unnecessary allocations.

• How can we reproduce it?
  Run a large azblob or azdatalake workload with profiling.

• Anything we should know about your environment.
  go version go1.25.6 darwin/arm64, however issue is version and architecture agnostic