feat(gax): add ResumableUploadClient.startUpload() and supporting types - #14138

🤖 I have created a release *beep* *boop*
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<details><summary>1.91.0</summary>

##
[1.91.0](v1.90.0...v1.91.0)
(2026-09-04)


### ⚠ BREAKING CHANGES

* **datalabeling:** remove java-datalabeling library
([#14189](#14189))
* **datacatalog:** remove java-datacatalog library
([#14178](#14178))

### Features

* **apptopology:** onboard a new library
([#14204](#14204))
([f871a07](f871a07))
* **bigtable:** Option to disable direct access fallback
([#14193](#14193))
([5b6a974](5b6a974))
* **datacatalog:** remove java-datacatalog library
([#14178](#14178))
([2910f63](2910f63))
* **datalabeling:** remove java-datalabeling library
([#14189](#14189))
([334d35a](334d35a)),
refs
[#14176](#14176)
* **gax:** add getSingleHeader to HttpHeadersUtils
([#14137](#14137))
([f114422](f114422))
* **gax:** add ResumableUploadClient.startUpload() and supporting types
([#14138](#14138))
([d7e2be2](d7e2be2))
* **gax:** add ResumableUploadResponseParser
([#14135](#14135))
([e1a0b17](e1a0b17))
* **gax:** allow non-JSON HttpContent and absolute request URLs in
HttpRequestRunnable
([#14134](#14134))
([ac4a49a](ac4a49a))
* **gax:** implement queryStatusCallable for resumable uploads
([#14155](#14155))
([67719c0](67719c0))
* **gax:** implement startUploadCallable for resumable uploads
([#14139](#14139))
([4e65b6c](4e65b6c))
* **gax:** implement uploadChunkCallable for resumable uploads
([#14140](#14140))
([1c643de](1c643de))
* **google/shopping/merchant/loyaltycustomers/v1:** onboard new library
([#14257](#14257))
([590a51c](590a51c))
* **grpc-gcp:** drain scaled-down channels
([#14216](#14216))
([bbbd18c](bbbd18c))
* **grpc-gcp:** move scale-up to background worker
([#14206](#14206))
([41f0a2e](41f0a2e))
* **grpc-gcp:** penalize retryable channel errors
([#14219](#14219))
([829872e](829872e))
* **grpc-gcp:** prime scaled channels before publish
([#14232](#14232))
([dd75645](dd75645))
* **spanner:** add client-level CallContextConfigurator to
SpannerOptions
([#14256](#14256))
([eef74d2](eef74d2))
* **spanner:** prime scaled-up dynamic channel pool channels with SELECT
1
([#14254](#14254))
([4d26645](4d26645))


### Bug Fixes

* **auth:** refine JSpecify nullability annotations for external account
credentials
([#14164](#14164))
([385e1f2](385e1f2))
* **bigquery-jdbc:** resolve `statementType` via `getJob` fallback to
avoid post-execution dry run for DDL
([#14265](#14265))
([d38d9f0](d38d9f0))
* **bigtable:** truncate client generated timestamps on the emulator c…
([#14234](#14234))
([94e680f](94e680f))
* **gax:** propagate structured LRO error details to ApiException
([#14022](#14022))
([865a15b](865a15b))
* **grpc-gcp:** correct channel lifecycle bookkeeping
([#14196](#14196))
([3df32ba](3df32ba))
* **grpc-gcp:** prevent hot-channel skew
([#14198](#14198))
([07a7505](07a7505))
* **java-sql:** temp fix for lint and skip generate
([#14005](#14005))
([b6f73db](b6f73db))
* **spanner:** prioritize leader replica for read-write transactions in
location-aware routing
([#14195](#14195))
([6383f81](6383f81))
* **spanner:** route ExecuteBatchDml through transaction affinity
endpoint
([#14192](#14192))
([5e9e3cd](5e9e3cd))
* **storage:** resolve GraalVM Native Image test failure for
java-storage
([#14226](#14226))
([93e0920](93e0920))


### Performance Improvements

* **bigquery-jdbc:** eliminate sync getJob RPC call
([#14197](#14197))
([ccf12b0](ccf12b0))
* **ci:** replace mvn help:evaluate with native bash and sed extraction
([#14218](#14218))
([744a522](744a522))


### Dependencies

* Upgrade grpc-java to 1.83.0
([#13967](#13967))
([0cdc695](0cdc695))
</details>

---
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Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>