Add support for DefaultAzureCredential in Azure Storage backend

[mkompanek]

Description

This change extends the Azure Storage (ASFileSystem) client initialization to support DefaultAzureCredential from the Azure Identity library.

• Introduces #include <azure/identity.hpp> and namespace ai = Azure::Identity;.
• Adds conditional initialization:
  • If AZURE_USE_DEFAULT_CREDENTIAL is set to "1" or "true", the BlobServiceClient is constructed using DefaultAzureCredential.
  • If account_key_ is provided, Shared Key authentication is used.
  • Otherwise, fallback to anonymous BlobServiceClient.

This enables Triton to authenticate with Azure Storage using Managed Identity, environment-based credentials, or developer tools (VS/CLI), improving integration with Azure-hosted deployments.

Testing

Built and ran Triton inside a container with Azure SDK support enabled.
Inside container I was able to successfully run:
`
az login

tritonserver --model-repository=as://deeplearningstorage.blob.core.windows.net/model-repo
`

FROM nvcr.io/nvidia/tritonserver:25.08-py3 AS base

FROM base AS build
ENV DEBIAN_FRONTEND=noninteractive
RUN apt-get update \
      && apt-get install -y --no-install-recommends \
         git cmake build-essential wget ca-certificates python3 python3-dev ninja-build pkg-config \
         zlib1g-dev libcurl4-openssl-dev libssl-dev zlib1g-dev uuid-dev pkg-config libkrb5-dev \
         libxml2-dev rapidjson-dev libboost-all-dev libre2-dev \
      && rm -rf /var/lib/apt/lists/*
RUN wget https://github.com/Kitware/CMake/releases/download/v3.31.8/cmake-3.31.8-linux-x86_64.tar.gz \
      && tar -xzf cmake-3.31.8-linux-x86_64.tar.gz -C /usr/local --strip-components=1 \
      && rm cmake-3.31.8-linux-x86_64.tar.gz

COPY . /workspace/core
WORKDIR /workspace/core
RUN mkdir build \
   && cd build \
   && cmake -DCMAKE_INSTALL_PREFIX=/workspace/install -DTRITON_CORE_HEADERS_ONLY=OFF -DTRITON_ENABLE_AZURE_STORAGE=ON .. \
   && make install

FROM base AS final
COPY --from=build /workspace/install/lib/libtritonserver.so /opt/tritonserver/lib
ENV TZ=Europe/Vienna
ENV DEBIAN_FRONTEND="noninteractive"
RUN apt-get update \
    && apt-get install --no-install-recommends -y bash curl \
    && curl -sL https://aka.ms/InstallAzureCLIDeb | bash \
    && curl -sL https://aka.ms/install-artifacts-credprovider.sh | bash \
    && rm -rf /var/lib/apt/lists/*

ENV AZURE_USE_DEFAULT_CREDENTIAL=1