Allow flags to be set with greater flexibility

[daveliddell]

Changes to the python binding to allow iree.runtime.flags.parse_flags to take effect at times other than before the first time a driver is created. Also includes fixes for bugs exposed during the development of this feature.

• Added "internal" API functions create_hal_driver() and clear_hal_driver_cache() to create a driver object independent of the cache, and to clear the cache, respectively
• Added HalDriver class implementation functions for the above new API functions. Refactored class to share as much common code as possible.
• Factored out driver URI processing into its own nested class for easier handling of URI components
• Fixed dangling pointer bug. In the C layer flags are being kept by reference as string views, requiring the caller to keep the original flag strings (argc, argv) around for as long as the flags are being used. However, the python binding was using a local variable for those strings, letting them go out of scope and causing garbage values later on. The fix is to move the strings to a file scope variable. Flag handling does not appear to be getting used in a multi-threaded environment, as other aspects of flag handling use static variables with no mutex guarding that I could find.
• Fixed runtime assert in Windows debug build for the improper use of std::vector<>::front() on an empty vector. The code never used the value of front(), as it was guarded by a check for the vector's size, but the assert prevents the debug build from running.

[daveliddell]

Please hold off reviewing; will rework client of this API not to use the driver cache, so will git rid of unnecessary new features

[daveliddell]

Snippet of ugly but functional use of this new feature:

class vmfbRunner:
    def __init__(self, device, vmfb_path, external_weight_path=None, extra_plugin=None):
        flags = []
        clean_driver = False
        if extra_plugin:
            ireert.flags.parse_flags(f"--executable_plugin={extra_plugin}")
            clean_driver = True
        haldriver = ireert.get_driver(device, clean_driver)

[daveliddell]

General note: in an offline discussion, we decided to keep the cache, so this PR is still a go. Making it ready-for-review again

[daveliddell]

New sample usage:

class vmfbRunner:
    def __init__(self, device, vmfb_path, external_weight_path=None, extra_plugin=None):
        flags = []

        # If an extra plugin is requested, add a global flag to load the plugin
        # and create the driver using the non-caching creation function, as
        # the caching creation function may ignore the flag.
        if extra_plugin:
            ireert.flags.parse_flags(f"--executable_plugin={extra_plugin}")
            haldriver = create_hal_driver(device)

        # No plugin requested: create the driver with the caching create
        # function.
        else:
            haldriver = ireert.get_driver(device)

[ScottTodd]

CI failure is preexisting and unrelated to this change, merging through it. https://github.com/iree-org/iree/actions/runs/9512652449/job/26221447494?pr=17659

[ RUN      ] NCCLDynamicSymbolsTest.CreateFromSystemLoader
iree/runtime/src/iree/hal/drivers/hip/dynamic_symbols_test.cc:92: Failure
Expected equality of these values:
  21803
  nccl_version
    Which is: 21806