[Identity] Support Azure Arc user-assigned managed identity (UAMI) in ManagedIdentityCredential

## Background

`azure-identity`'s `ManagedIdentityCredential` supports Azure Arc, but **only system-assigned managed identity (SAMI)**; user-assigned managed identity (UAMI) is explicitly blocked. The library implements Arc **natively** (there is no MSAL for C++), so — unlike the MSAL-based Azure SDKs — enabling Arc UAMI requires a native code change in this repo (it will not arrive via any dependency bump).

Azure Arc now supports UAMI at the platform level. This issue tracks enabling it in `azure-identity`.

## Current behavior

- `AzureArcManagedIdentitySource::Create` (`sdk/identity/azure-identity/src/managed_identity_source.cpp`) throws when a user-assigned id is supplied:
> User assigned identity is not supported by the Azure Arc Managed Identity Endpoint. To authenticate with the system assigned identity, omit the client, object, or resource ID when constructing the ManagedIdentityCredential.
- The `AzureArcManagedIdentitySource` constructor passes an **empty** client id to the base `ManagedIdentitySource` and only appends `api-version=2019-11-01`; it never appends a user-assigned id query parameter (unlike `AppServiceManagedIdentitySource`, which appends `client_id` / `principal_id` / `mi_res_id`).
- The challenge-response path (`ValidateArcKeyFile`, `WWW-Authenticate` handling) is otherwise complete and identical for SAMI/UAMI.

## Scope of work

- [ ] Remove the UAMI `throw` in `AzureArcManagedIdentitySource::Create`; pass `clientId` / `objectId` / `resourceId` into the constructor.
- [ ] Update the `AzureArcManagedIdentitySource` constructor (and its declaration in `sdk/identity/azure-identity/src/private/managed_identity_source.hpp`) to forward a non-empty client id to the base and append the correct id query parameter (verify names — Arc/IMDS `msi_res_id` vs App Service `mi_res_id`).
- [ ] No change needed in `GetToken` — both the initial and challenge-response requests clone `m_url`, so the id parameter flows through automatically.
- [ ] Update/add unit tests (`ManagedIdentityCredentialTest`): flip the existing "Arc + UAMI throws" case and assert the request URL carries the id parameter.
- [ ] Update `CHANGELOG.md` and docs (remove Arc SAMI-only wording).

## Notes

- Requires the Arc HIMDS endpoint to accept a user-assigned id (the platform prerequisite; the same one the MSAL GA depends on).
- This is a **native (non-MSAL)** change; it does not depend on any MSAL version.
- The C SDK (`azure-sdk-for-c`) has no managed-identity credential, so no equivalent change applies there.

## Key references

- `sdk/identity/azure-identity/src/managed_identity_source.cpp`
- `sdk/identity/azure-identity/src/private/managed_identity_source.hpp`
- `sdk/identity/azure-identity/src/managed_identity_credential.cpp`

Start with AzureArcManagedIdentitySource::Create in sdk/identity/azure-identity/src/managed_identity_source.cpp and its declaration in private/managed_identity_source.hpp, then inspect the ManagedIdentityCredentialTest Arc UAMI case. Run the identity unit tests and verify that Arc UAMI no longer throws and that the request URL carries the appropriate id parameter. Update CHANGELOG.md and the documentation to remove Arc SAMI-only wording.

Written by the indexing model from the issue text.